Looks like there are still some bugs open[0] and some unimplemented details[1]. Self promo of my arewemodulesyet to see the status of modules support in the c++ library ecosystem [2].

[0] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103524

[1] https://gcc.gnu.org/projects/cxx-status.html#cxx20

[2] https://arewemodulesyet.org/