A couple of ideas

[superwills]

Your library seems very promising, but I thought I would let you know a few ideas I had while looking at it:

• Would be really great to get https support
• I'd request to avoid the use of C++ exceptions. Many C++ devs don't use them very much (they're notoriously expensive!)
• The single header, although it looks "easier to use" at first, is really difficult to browse. Splitting the source file into multiple files is almost always the best idea.
• style nit: I feel like you're overusing auto in the code. I mean, in this case, int is shorter ;) The general guideline I've heard is to use auto where it "increases readability", very subjective :)
• There is some extra data copy that could be avoided. Consider going to C-arrays/memcpy for passing around some of the data?
• Would be nice to have an asyncSend that accepts a lambda onComplete handler (something like std::function<void (Response)>)

[superwills]

See also this softwareengineering.stackexchange answer on exceptions being considered an antipattern

[Nevermore1994]

I rewrote the HTTP request.
https://github.com/Nevermore1994/http-request

In this repo,It sounds like all the ideas you mentioned have already been implemented.

By the way, auto is quite beneficial for C++ development. Using auto extensively does not increase the cognitive load for developers; on the contrary, it can significantly reduce the burden on both developers and the compiler.