> From a conceptual point of view I think that specific wals per subscription > should be used and stored in the pg_replslot folder in order to avoid > working directly on the wals of the instance. > What do you think about this proposal?
I think that would open a wholly new can of worms. The most obvious point here is: that WAL is primarily generated for the operation of the database itself - it's our kind of transaction log, or "Redo Log" in other systems' lingo. Replication (be it physical or logical) is a secondary purpose (an obvious and important one, but still secondary). How would you know which part of WAL is needed for any specific replication slot? You'd have to decode and filter it, and already you're back at square one. How would you handle multiple replications for the same table (in the same publication, or even over multiple (overlapping) publications) - do you multiply the WAL?
For now, we have "any replication using replication slots, be it logical or physical replication, retains WAL up to max_slot_wal_keep_size (or "unlimited" if not set - and on PostgreSQL 12 and before); and you need to monitor the state of your replication slots", which is a totally usabe rule, I think.