As far as I know it's impossible to reliably pg_upgrade a node that has subscriptions and eventually resume logical replication.
Should this go in the documentation somewhere? Maybe in the pg_upgrade notes? I still don't understand the mechanism. You also say that:
It's possible to make it work with some efforts in some basic configurations and / or if no changes happen on the publications
But that kind of surprises me too, actually, because it seemed like pg_upgrade wiped out the LSN locations of the subcriber, making it start all over.
Upgrading a subscriber seems like something that could/should work, so it should be documented if pg_upgrade is incompatible with maintaining a subscription, shouldn't it?
