I don't see this as a big issue, unless I'm misunderstanding.
Personally I would call it a deal-breaker if I thought it'd affect
more than a very very tiny number of people. But the entire premise
of this patch is that nobody is using standard_conforming_strings =
off in production anymore. If that isn't true it's probably a
mistake to go forward anyway.
I think you're probably right that very few people are using it. Not sure how we'd find out if they are, though.
I've poked around for a while now, and I can't find anything that should have been removed that wasn't already removed.
I'm a teensy bit uneasy that we're outright removing escape_string_warning, perhaps allowing it to be set but not actually setting the GUC might allow some ancient script to avoid an error, but that unease isn't enough to stop anything.
I do think we should make an effort in the release notes to say that db-dumps with standard_conforming_strings = off must first be restored to a pre-19 instance before upgrading, but that's about it.
