I think we should not delete them from the synopsis as they are used in pre C++20 mode. You could mark them with until C++17 and then spaceship with C++20

I think you are missing this specialization below. That said, I am unsure whether this is actually needed at all.

That sounds reasonable. Thanks!

Shouldn't notes like "(Until C++17)" be just trailing comments?

I may be missing something, but it seems that there are no tests for the basic cases like:
"abc" < "acb"
"cba" > "aaa"
All (non-equal) tests fall into the shorter/longer check.

You changed the order of parameters and now this overload is the same as the one below.
And BTW, why removing spaces in <...>? :)

I think you should mark the overloads removed in C++20 as removed in C++20 or until C++17 instead of just C++17 as the latter is misleading.

Return type is not bool -> see below in standard.

This overload doesn't seem to be marked noexcept by the standard: http://eel.is/c++draft/string.syn.

Where is the return type? Standard marks it as see below.

I think you should just use noexcept instead of _NOEXCEPT as it should be always available in C++20.

This _NOEXCEPT should be kept as is of course.

bool?

Great!