They were already added (in the std namespace) in C++20 (see [cuchar.syn] and [c.mb.wcs] p7-11).
Do we want these in C++26?
If we rebase C++ on C2x we'll pick these up as part of <cuchar>. I assume that's OK.
Yes, that is expected. We will need to adjust the wording for the linked sections above when we rebase:
The contents and meaning of the header <cuchar> are the same as the C standard library header <uchar.h>, except that it
declares the additional mbrtoc8 and c8rtomb functions anddoes not declare types char8_t, char16_t,nor char32_t.
Align the specification for these functions with the other functions in that section to give them the "These functions have the semantics specified in the C standard library" treatment with appropriate "See also: ISO/IEC 9899:..." notes.
Tom.