I never used modules, but I heard about them. I just checked the official checklist from Clang, and I see this: https://clang.llvm.org/cxx_status.html#cxx20

Wow, the list of papers for Modules is large. I can understand why it is taking so long to implement. Also, there needs to be "fingers to do the typing". I assume most of the commercial devs (Google, Apple) just don't think modules are important enough. It looks like Concepts also took a very long time to be fully implemented. Hell, that feature was discussed for 10 years (a dream of Bjarne) before approved into a standard... then maybe 5 years to impl!