I recently had to cut loose (pg_drop_replication_slot) a logical replica that couldn't keep up and so was threatening to bring down the master.
In mopping up on the replica side, I couldn't just drop the subscription, because it couldn't drop the nonexistent slot on the master and so refused to work. So I had to do a silly little dance where I first disable the subscription, then ALTER SUBSCRIPTION ... SET (slot_name = NONE), then drop it.
Wanting to clean up after itself is admirable, but if there is nothing to clean up, why should that be an error condition? Should this be an item on https://wiki.postgresql.org/wiki/Todo (to whatever extent that is still used).
