Likely the existence and explicit mention of os.fsencode, as well as the api giving what you put in, so accepting bytes if you do things safely.

The 2 API also returns the same type you put in.

os.fsencode() was changed in Python 3 longer after the 3.0 fork and isn't inherent to 3 -- the same change could be made to Python 2. It only wasn't because Python.org has been intentionally neglecting Python 2 since 2015.

(The Python 2 equivalent of os.fsencode() is just encoding with sys.getfilesystemencoding(). The primary difference is that fsencode() uses the surrogateescape error handler by default. Since this is a relaxed behavior, it seems like it could be added to Python 2 without regressing existing programs.)

Yes, so the documentation and language explicitly have tools to avoid encoding issues and make you aware of them. It encourages writing good, unbroken, code.