Well, if that is the standard for portability then may_alias might as well be standard. GCC and Clang support it and MSVC doesn't implement the affected optimization as far as I can find.
Within the context of this discussion portability was mentioned as key feature of the standard. If C23 adoption is as limited as the, possibly outdated, tables on cppreference and your comments about gcc, clang and msvc suggest then the functionality provided by the gcc attribute would be more portable than C23 conformant code. You could call it a de facto standard, as opposed to C23 which is a standard in the sense someone said so.