There’s one aspect of the versioning/transition issue that is directly relevant to this PEP.
Given that the PEP explicitly notes that things like path dependencies are deferred for future revisions, how should this PEP advise tools to handle such “future extensions” if they encounter them? If we don’t answer that question we are just repeating the same mistake that we made with the [project] table. And while I’m sympathetic to the view that this is an issue that needs to be solved by versioning pyproject.toml as a whole, we still need to decide what to do before then (even if it’s “extensions are prohibited until versioning is solved”).