I really want to focus on the first patch, timeline following for logical slots. That part is much less invasive and is useful stand-alone. I'll move it to a separate CF entry and post it to a separate thread as I think it needs consideration independently of failover slots.
Just an update on the failover slots status: I've moved timeline following for logical slots into its own patch set and CF entry and added a bunch of tests.
Once some final changes are made to the tests for timeline following I'll address the checkpoint issue in failover slots by doing the checkpoint of slots at the start of a checkpoint/restartpoint, while we can still write WAL. Per the comments in CheckPointReplicationSlots it's mostly done in a checkpoint currently for convenience.
Then I'll write some TAP tests for failover slots and submit an updated patch for them, by which time hopefully timeline following for logical slots will be committed.
In other words this patch isn't dead, the foundations are just being rebased out from under it.