I replied to INADA-san message on bpo-43552:
https://bugs.python.org/issue43552#msg389091
> I had forgot to consider about UTF-8 mode while finishing PEP 597. If possible, I want to ignore UTF-8 mode when `encoding="locale"` is specified from Python 3.10.
In this case, the PEP 597 statement that open(filename, encoding="locale") is the same than open(filename) is wrong. It would mean that users which got the UTF-8 Mode enabled (implicitly or explicitly) would switch to a legacy encoding like latin1 rather than using the UTF-8 encoding, if they add encoding="locale" to their open() calls?
Since the final goal is to move everybody towards to UTF-8, I'm not sure how it's a good thing.