We just now deployed new PR templates with our review checklists that authors will see prior to submitting their PRs (as well as after); the first item for Standards-Track PEPs asks whether the PEP has already been discussed, refined, and generally agreed to be ready in an appropriate discussion venue first, and another item asks that the PEP has a sponsor who has confirmed they are okay with it first, if not authored by a core dev or PEP editor. This will hopefully help guide new PEP authors toward ensuring their PEPs are complete before posting them.
Initially, in the new checklist, that was the case. However, after multiple others suggested such, to simplify the process for both authors and reviewers, we tweaked it so new PEP authors are now instructed to just use the next available sequential number for their PEP instead of having one manually assigned post-facto, such that there’s no need to go back through and change everything assuming the PEP is merged and there’s no number conflict. However, as before, the number is not actually assigned to them until the PEP is merged, and is double-checked as part of the PEP editor review, and the checklist only recommends creating the official PEP discussion thread just before or just after it is actually merged.