Re: Standby server with cascade logical replication could not be properly stopped under load

nd master versions.

Yeah, I think that this is a sensible move.  Documenting the reason
why GetXLogReplayRecPtr() is called is important, but the comment you
are suggesting could be better, IMO, say:
"For cascading logical WAL senders, we use the replay LSN rather than
the flush LSN, as decoding would only happen with records already
replayed.  This distinction is important especially during the
shutdown sequence, as cascading logical WAL senders can only catch up
with records that have been already replayed, not flushed."

That feels that I'm repeating myself a bit twice if formulated this
way.  If you have a better suggestion, feel free..