Announce planned 4.0 deprecations

[daverodgman]

Mark things as deprecated in 3.x, so that we can remove things in 4.0.

This issue needs a list of what exactly we will deprecate before we can work on it; for now it's a placeholder.

[gilles-peskine-arm]

The discussion of what to remove in 4.0 is happening at #6792.

Note that there are likely things we want to change — not removing functionality, but changing the way to achieve the same functionality. But we don't particularly need to deprecate the old version in advance for that (although we certainly can, and given the number of things we want to change in 4.0 we should get started on those early).

[mpg]

See also #8663 for a (provisional) list of things we want to make internal, which should be announced as well (from a user's point of view, it's the same as a removal).

[mpg]

See https://lists.trustedfirmware.org/archives/list/mbed-tls-announce@lists.trustedfirmware.org/thread/YREHWMX4EAHMTMGOUZUVDVOQBHHSNXUL/

GitHub links — cryptographic mechanisms
Mbed-TLS/TF-PSA-Crypto#102 — Custom ECC mechanisms
Mbed-TLS/TF-PSA-Crypto#104 — Custom RSA mechanisms
Mbed-TLS/TF-PSA-Crypto#105 — Import of incomplete RSA private keys
Mbed-TLS/TF-PSA-Crypto#107 — Direct access to CTR_DRBG and HMAC_DRBG
#8459 — RSA PKCS#1v1.5 encryption
#9164 — DES (including 3DES)

GitHub links — cryptography implementations
Mbed-TLS/TF-PSA-Crypto#103 — Partial ECC acceleration
#8151 — Dynamically registered secure element drivers

GitHub links — TLS 1.2
#5278 — FFDH in TLS 1.2
#8170 — RSA decryption cipher suites (RSA without DH/ECDH)
#9201 — Static ECDH cipher suites
#9202 — CBC cipher suites
#9201
GitHub links — platform
#8108 — Platform interface redesign
Mbed-TLS/TF-PSA-Crypto#106 — Building with plain make or CMake
#8231 — x86_64: AESNI without compiler intrinsics
#9307 — Support for %zu in printf

[gilles-peskine-arm]

For 3.x, we missed the ball and never announced any of the deprecations.

For 4.0, we've sent consultation emails. I expect that we'll send other messages once we've made decisions, but there's no particular timeline on announcements. Some aspects of the 4.0 design will not be finalized until March 2025, when we plan remaining changes based on the available time. So I consider this issue to be done.