Hello, Peter!
> FWIW _hash_readpage has a comment about a stashed LSN, so it seems as
> if this was barely missed by the work on hash indexes around 2017:
I think commit 22c5e735 [0] (Remove lsn from HashScanPosData) is the
thing you are looking for in relation to hash.
Best regards,
Mikhail.
[0]: https://github.com/postgres/postgres/commit/22c5e73562c53437979efec4c26cd9fff408777c