But the byte '/' can never be part of any filename/dirname under a UNIX filesystem. Which kinda sucks generally for anyone wanting to use a charset like that, but doesn't it also mean that should never be a problem for `dirname()`?

I'm struggling to imagine how this failure would manifest. Can you give an example of how dirname() would fail? What combination of existing file/directory name, and usage of that function, would not work as expected?

Edit: I'm also a bit confused how this counts as being a problem for "modern Linux systems" - wouldn't it have always been a problem for all Unix-based OSs?