I actually do not think this is useful. Quite the opposite. Around 50% of our uses are of the following form:

int bar(int baz)
{
#ifdef SOMETHING
 return foobar(baz);
#else
 MPT_UNREFERENCED_PARAMETER(baz);
 return 23;
#endif
}

[[maybe_unused]] instead would be used in the variable declaration, which would actually hide accidentally not using baz in the first preprocessor branch.

I still do not think this would be very useful.

If anything, I would prefer a form of this feature that would actually warn if used in the opposite case when applied to a variable that actually does get used regardless. Our current macro does theoretically semantically allow for such a future warning, while [[maybe_unused]] does prohibit any warning in this case.