The July Commitfest has started. This patch is in "Needs review" status, but it doesn't apply. If I read the above discussion correctly, it seems there is agreement that a warning here is a good idea to commit this patch. Could you please post a rebased patch?
I followed your suggestion and changed the message and added HINT. I hope everything is agreed now.
I wonder if it would be more typical project style to put the clue on what to do into an "errhint" message, something like this:
WARNING: insufficient wal_level to publish logical changes HINT: Set wal_level to logical before creating subscriptions.
