Why not put a notice at
the top of the PEP indicating that it was accurate when it was accepted,
but that changed have happened since then, see the docs for details?
I see PEP as the design document and docs as user-friendly documentation. It helps to keep both of them up to date and in sync.
If an "Accepted" PEP is not updated, then folks landing on it will feel misguided or confused if the read the docs later. Presenting a cohesive story here is probably more work than simply keeping them in sync.
The history of decision changes is tracked in VCS, if that is desirable.