A new LWG issue was filed for this question today:
- https://cplusplus.github.io/LWG/issue3314

This issue concerns the ostream inserters added for std::chrono::duration in C++20 and what the intended behavior is for a duration when period::type is micro.

[time.duration.io]p4 states:

If Period​::​type is micro, but the character U+00B5 cannot be represented in the encoding used for charT, the unit suffix "us" is used instead of "μs".

The question is with regard to which one of the encodings used for charT is referred to here; the compile-time execution character set or the run-time locale dependent native character set?

The proposed resolution specifies that the compile-time execution character set is the intended one.  My expectation is that this aligns with existing implementations, but I haven't checked.