> Path names are UTF-8-encoded, unrooted, slash-separated sequences of path elements, like “x/y/z”

This is only for the "io/fs" package and its generic filesystem abstractions. The "os" package, which always operates on the real filesystem, doesn't actually specify how paths are encoded, nor does its associated helper package "path/filepath".

In practice, non-UTF-8 already wasn't an issue on Unix-like systems, where file paths are natively just byte sequences. You do need to be aware of this possibility to avoid mangling the paths yourself, though. The real problem was Windows, where paths are actually WTF-16, i.e. UTF-16 with unpaired surrogates. Go has addressed this issue by accepting WTF-8 paths since Go 1.21: https://github.com/golang/go/issues/32334#issuecomment-15500...