diff --git a/src/poetry/installation/executor.py b/src/poetry/installation/executor.py
index 9a2aad10734..c0580bba2e6 100644
--- a/src/poetry/installation/executor.py
+++ b/src/poetry/installation/executor.py
@@ -68,6 +68,17 @@ def __init__(
         self._use_modern_installation = config.get(
             "installer.modern-installation", True
         )
+        if not self._use_modern_installation:
+            self._io.write_line(
+                "<warning>Setting `installer.modern-installation` to `false` "
+                "is deprecated.</>"
+            )
+            self._io.write_line(
+                "<warning>The pip-based installer will be removed in a future release.</>"
+            )
+            self._io.write_line(
+                "<warning>See https://github.com/python-poetry/poetry/issues/8987.</>"
+            )
 
         if parallel is None:
             parallel = config.get("installer.parallel", True)
diff --git a/tests/installation/test_executor.py b/tests/installation/test_executor.py
index d73705ff405..7b5e6e22b9e 100644
--- a/tests/installation/test_executor.py
+++ b/tests/installation/test_executor.py
@@ -206,16 +206,18 @@ def test_execute_executes_a_batch_of_operations(
         develop=True,
     )
 
-    return_code = executor.execute([
-        Install(Package("pytest", "3.5.1")),
-        Uninstall(Package("attrs", "17.4.0")),
-        Update(Package("requests", "2.18.3"), Package("requests", "2.18.4")),
-        Update(Package("pytest", "3.5.1"), Package("pytest", "3.5.0")),
-        Uninstall(Package("clikit", "0.2.3")).skip("Not currently installed"),
-        Install(file_package),
-        Install(directory_package),
-        Install(git_package),
-    ])
+    return_code = executor.execute(
+        [
+            Install(Package("pytest", "3.5.1")),
+            Uninstall(Package("attrs", "17.4.0")),
+            Update(Package("requests", "2.18.3"), Package("requests", "2.18.4")),
+            Update(Package("pytest", "3.5.1"), Package("pytest", "3.5.0")),
+            Uninstall(Package("clikit", "0.2.3")).skip("Not currently installed"),
+            Install(file_package),
+            Install(directory_package),
+            Install(git_package),
+        ]
+    )
 
     expected = f"""
 Package operations: 4 installs, 2 updates, 1 removal
@@ -316,24 +318,26 @@ def test_execute_prints_warning_for_invalid_wheels(
     base_url = "https://files.pythonhosted.org/"
     wheel1 = "demo_invalid_record-0.1.0-py2.py3-none-any.whl"
     wheel2 = "demo_invalid_record2-0.1.0-py2.py3-none-any.whl"
-    return_code = executor.execute([
-        Install(
-            Package(
-                "demo-invalid-record",
-                "0.1.0",
-                source_type="url",
-                source_url=f"{base_url}/{wheel1}",
-            )
-        ),
-        Install(
-            Package(
-                "demo-invalid-record2",
-                "0.1.0",
-                source_type="url",
-                source_url=f"{base_url}/{wheel2}",
-            )
-        ),
-    ])
+    return_code = executor.execute(
+        [
+            Install(
+                Package(
+                    "demo-invalid-record",
+                    "0.1.0",
+                    source_type="url",
+                    source_url=f"{base_url}/{wheel1}",
+                )
+            ),
+            Install(
+                Package(
+                    "demo-invalid-record2",
+                    "0.1.0",
+                    source_type="url",
+                    source_url=f"{base_url}/{wheel2}",
+                )
+            ),
+        ]
+    )
 
     warning1 = f"""\
 <warning>Warning: Validation of the RECORD file of {wheel1} failed.\
@@ -426,9 +430,11 @@ def test_execute_works_with_ansi_output(
 
     executor = Executor(env, pool, config, io_decorated)
 
-    return_code = executor.execute([
-        Install(Package("cleo", "1.0.0a5")),
-    ])
+    return_code = executor.execute(
+        [
+            Install(Package("cleo", "1.0.0a5")),
+        ]
+    )
 
     # fmt: off
     expected = [
@@ -461,9 +467,11 @@ def test_execute_works_with_no_ansi_output(
 
     executor = Executor(env, pool, config, io_not_decorated)
 
-    return_code = executor.execute([
-        Install(Package("cleo", "1.0.0a5")),
-    ])
+    return_code = executor.execute(
+        [
+            Install(Package("cleo", "1.0.0a5")),
+        ]
+    )
 
     expected = """
 Package operations: 1 install, 0 updates, 0 removals
@@ -626,12 +634,14 @@ def test_executor_should_not_write_pep610_url_references_for_cached_package(
     io: BufferedIO,
 ) -> None:
     link_cached = fixture_dir("distributions") / "demo-0.1.0-py2.py3-none-any.whl"
-    package.files = [{
-        "file": "demo-0.1.0-py2.py3-none-any.whl",
-        "hash": (
-            "sha256:70e704135718fffbcbf61ed1fc45933cfd86951a744b681000eaaa75da31f17a"
-        ),
-    }]
+    package.files = [
+        {
+            "file": "demo-0.1.0-py2.py3-none-any.whl",
+            "hash": (
+                "sha256:70e704135718fffbcbf61ed1fc45933cfd86951a744b681000eaaa75da31f17a"
+            ),
+        }
+    ]
 
     mocker.patch(
         "poetry.installation.executor.Executor._download", return_value=link_cached
@@ -653,12 +663,14 @@ def test_executor_should_write_pep610_url_references_for_wheel_files(
     url = (fixture_dir("distributions") / "demo-0.1.0-py2.py3-none-any.whl").resolve()
     package = Package("demo", "0.1.0", source_type="file", source_url=url.as_posix())
     # Set package.files so the executor will attempt to hash the package
-    package.files = [{
-        "file": "demo-0.1.0-py2.py3-none-any.whl",
-        "hash": (
-            "sha256:70e704135718fffbcbf61ed1fc45933cfd86951a744b681000eaaa75da31f17a"
-        ),
-    }]
+    package.files = [
+        {
+            "file": "demo-0.1.0-py2.py3-none-any.whl",
+            "hash": (
+                "sha256:70e704135718fffbcbf61ed1fc45933cfd86951a744b681000eaaa75da31f17a"
+            ),
+        }
+    ]
 
     executor = Executor(tmp_venv, pool, config, io)
     executor.execute([Install(package)])
@@ -687,12 +699,14 @@ def test_executor_should_write_pep610_url_references_for_non_wheel_files(
     url = (fixture_dir("distributions") / "demo-0.1.0.tar.gz").resolve()
     package = Package("demo", "0.1.0", source_type="file", source_url=url.as_posix())
     # Set package.files so the executor will attempt to hash the package
-    package.files = [{
-        "file": "demo-0.1.0.tar.gz",
-        "hash": (
-            "sha256:9fa123ad707a5c6c944743bf3e11a0e80d86cb518d3cf25320866ca3ef43e2ad"
-        ),
-    }]
+    package.files = [
+        {
+            "file": "demo-0.1.0.tar.gz",
+            "hash": (
+                "sha256:9fa123ad707a5c6c944743bf3e11a0e80d86cb518d3cf25320866ca3ef43e2ad"
+            ),
+        }
+    ]
 
     executor = Executor(tmp_venv, pool, config, io)
     executor.execute([Install(package)])
@@ -796,12 +810,14 @@ def test_executor_should_write_pep610_url_references_for_wheel_urls(
         source_url="https://files.pythonhosted.org/demo-0.1.0-py2.py3-none-any.whl",
     )
     # Set package.files so the executor will attempt to hash the package
-    package.files = [{
-        "file": "demo-0.1.0-py2.py3-none-any.whl",
-        "hash": (
-            "sha256:70e704135718fffbcbf61ed1fc45933cfd86951a744b681000eaaa75da31f17a"
-        ),
-    }]
+    package.files = [
+        {
+            "file": "demo-0.1.0-py2.py3-none-any.whl",
+            "hash": (
+                "sha256:70e704135718fffbcbf61ed1fc45933cfd86951a744b681000eaaa75da31f17a"
+            ),
+        }
+    ]
 
     executor = Executor(tmp_venv, pool, config, io)
     operation = Install(package)
@@ -890,12 +906,14 @@ def mock_get_cached_archive_func(
         source_url="https://files.pythonhosted.org/demo-0.1.0.tar.gz",
     )
     # Set package.files so the executor will attempt to hash the package
-    package.files = [{
-        "file": "demo-0.1.0.tar.gz",
-        "hash": (
-            "sha256:9fa123ad707a5c6c944743bf3e11a0e80d86cb518d3cf25320866ca3ef43e2ad"
-        ),
-    }]
+    package.files = [
+        {
+            "file": "demo-0.1.0.tar.gz",
+            "hash": (
+                "sha256:9fa123ad707a5c6c944743bf3e11a0e80d86cb518d3cf25320866ca3ef43e2ad"
+            ),
+        }
+    ]
 
     executor = Executor(tmp_venv, pool, config, io)
     operation = Install(package)
@@ -1163,12 +1181,20 @@ def test_executor_fallback_on_poetry_create_error_without_wheel_installer(
         "poetry.factory.Factory.create_poetry", side_effect=RuntimeError
     )
 
-    config.merge({
-        "cache-dir": str(tmp_path),
-        "installer": {"modern-installation": False},
-    })
+    config.merge(
+        {
+            "cache-dir": str(tmp_path),
+            "installer": {"modern-installation": False},
+        }
+    )
 
     executor = Executor(env, pool, config, io)
+    warning_lines = io.fetch_output().splitlines()
+    assert warning_lines == [
+        "Setting `installer.modern-installation` to `false` is deprecated.",
+        "The pip-based installer will be removed in a future release.",
+        "See https://github.com/python-poetry/poetry/issues/8987.",
+    ]
 
     directory_package = Package(
         "simple-project",
@@ -1177,9 +1203,11 @@ def test_executor_fallback_on_poetry_create_error_without_wheel_installer(
         source_url=fixture_dir("simple_project").resolve().as_posix(),
     )
 
-    return_code = executor.execute([
-        Install(directory_package),
-    ])
+    return_code = executor.execute(
+        [
+            Install(directory_package),
+        ]
+    )
 
     expected = f"""
 Package operations: 1 install, 0 updates, 0 removals
@@ -1485,10 +1513,12 @@ def test_other_error(
             },
         ),
         (
-            [{
-                "file": "demo-0.1.0.tar.gz",
-                "hash": "md5:d1912c917363a64e127318655f7d1fe7",
-            }],
+            [
+                {
+                    "file": "demo-0.1.0.tar.gz",
+                    "hash": "md5:d1912c917363a64e127318655f7d1fe7",
+                }
+            ],
             {
                 "archive_info": {
                     "hashes": {"md5": "d1912c917363a64e127318655f7d1fe7"},