A POSIX filename is a sequence of up to 255 bytes excluding NULL and /

A reasonable person[1] might argue that this is too liberal. IMO the exclusion list should be larger, but silently and automatically rewriting an application's filename into something else at creation time is too far.

[1]: https://dwheeler.com/essays/fixing-unix-linux-filenames.html

No, but I read "POSIX norms" as what commonly happens on Unix systems: the filenames you get back are what you put into it.

Most of them don't concern over character encoding and UTF-8 sequences are just as-is. ZFS is again the special case, the normalization property will normalize names on lookup (they are stored as-is).