>> I'm not sure why this is different from any other feature-test macro.
>> [...]
>
> What would be the use case for this?

Well, certainly some feature-test macros indicate the presence of significant portable functionality: __cpp_lib_ranges_iota, for example. It is equally certain that feature-test macros are of less value when they indicate the presence of things that do not have such portable functionality. Set against this is the questionable value that a "really does what I ask" test has in a context where "really" isn't very meaningful.

Davis