I'm following up to see if anyone else would categorize this as a bug. It renders my old primaries dead until I do a restore on them.
No, I don’t think so. When you promote a standby; the primary will have to be restored or rewinded using pg_rewind to turn it into a replica. Another approach is to cleanly shutdown the primary and then you can turn it into a replica without doing a restore or pg_rewind.
Just to be clear if it wasn’t already self evident by my above statement: The primary needs to be clearly shutdown (all WALs archived), apply all WALs to replica, promote the replica and then the original primary can be turn into a replica without a restore or pg_rewind.