The restrictions aren't, but I think TOCs can and do set barriers to reject *all* off peak tickets at certain times, with filtering then done manually.
Indeed. I had the outward portion of a Stevenage - Carlisle off-peak return route:+Any Permitted rejected by the gates at Finsbury Park this morning.
There seemed to be a lot of revenue activity on Great Northern this morning so I suspect that had something to do with it.