We ran into this too, i actually think its a terrible postgres default. Logical replication slots should have timeouts where if you haven't read the WAL record in say 24 hours it should be dropped. Make it configurable and set a sane default, problem solved.

You'd have to resync the followers/secondaries, big deal, its way better than the primary going down because its disk filled up. This failure mode is awful. On RDS its relatively painless because you can snap fingers and have more disk but if you are running it yourself? Good luck.

In practice, mongodb’s oplog mechanism, for example, which acts as a circular buffer with a set size is a much more tolerant implementation. If the oplog rolls over befrore you've read it just resync but at most its taken up 10% of your disk.