OK, the PR is up and I’ve updated OP to reflect that.
This is intended to be an alternative to PEP 582, solves the same user-concerns while also resolving the question of how to pick for a virtualenv directory name by literally picking one to recommend by default, as a convention.
This behaviour change was briefly discussed in Speculative: `--require-venv` by default · Issue #10833 · pypa/pip · GitHub, however this PEP takes a different angle from that discussion: providing a consistent cross-platform experience that also eliminates potential of mistakes/confusion due not activating a virtual environment.