It is difficult to tell when this can happen but you detailed there is a theoretical possibility of the same. If we had an in-core cluster tool that manages nodes on its own which doesn't allow such scenarios to happen then we could possibly say that using such a tool it is safe to overwrite old primary's slots.
That's a lot of ifs, and none of them could be fulfilled in the foreseeable future.
Situation you describe is impossible.
When there is a split-brain and someone drops and re-creating logical slots with the same names on the old primary - such node can't be joined as a standby without pg_rewind.
In its current state pg_rewind wipes the pg_replslot directory, and therefore there will be no replication slots.
That is, if there is a logical replication slot with failover=true and synced=false on a healthy standby, it could have happened only because the old primary was shut down gracefully.
