Not propagating them through the WAL also has the rather large advantage of not barring the way to using such slots on standbys.
Yeah. So you could have a read-replica that has a slot and it has child nodes you can fail over to, but you don't have to have the slot on the master.
I don't personally find that to be a particularly compelling thing that says "we must have this" ... but maybe I'm not seeing the full significance/advantages.
I think it's technically quite possible to maintain the required resources on multiple nodes. The question is how would you configure on which nodes the resources need to be maintained? I can't come up with a satisfying scheme...
That's part of it. Also the mechanism by which we actually replicate them - protocol additions for the walsender protocol, how to reliably send something that doesn't have an LSN, etc. It might be fairly simple, I haven't thought about it deeply, but I'd rather not go there until the basics are in place.
