Easy, go hunting their manuals for the compilers that aren't yet another clang fork, still a few around between TI, Microchip and co on embedded.

In general, not for Qt deployment scenarios, also take into consideration the surviving big iron UNIX, mainframes and micros still in business, again the compilers that are yet not clang forks.

The table is a non exaustive overview, however it also paints the picture that for those that care about real portable code, regular visits to the table are required before considering adopting any specific new feature, or the need to enforce style guides like "we use C++20, but feature X, Y, Z are forbidden due to lack of portablity".