There has always been a rule that Cabin crew should not sleep in passenger cabins, at least for the almost quarter of a century that I have been flying! What has varied has been the punishment handed out for transgressions. I can remember a SCCM being stripped of rank as long ago as 1990ish for sleeping in First. Personally the only exception to this I have made is when I have a crew member who is unwell and then only after consultation with the skipper or when ferrying an empty aircraft when all rules are suspended!