Here's v2 of the failover slots patch. It replicates a logical slot to a physical streaming replica downstream, keeping the slots in sync. After the downstream is promoted a client can replay from the logical slot.
UI to allow creation of non-failover slots is pending.
There's more testing to do to cover all the corners: drop slots, drop and re-create, name conflicts between downstream !failover slots and upstream failover slots, etc.
There's also a known bug where WAL isn't correctly retained for a slot where that slot was created before a basebackup which I'll fix in a revision shortly.
I'm interested in ideas on how to better test this.