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 <http://eel.is/c++draft/time.duration.io#4> 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.

Tom.