WAL files not being deleted and pg_drop_replication_slot('barman') causes [55000] ERROR: replication slots can only be used if wal_level > = archive
Set "wal_level" back to "archive", and restart.  Then drop the slot.  Then if you don't want leave it at "archive", set it back to "minimal" again, set max_replication_slots to 0, and restart again.


  Which I find strange because I'm trying to remove it, not use it.


Removing a slot requires access to the slot machinery.  It is a form of "use".