Yawn's potential as a problematic psudo-shuffling mechanic, forcing the opponent to either switch out every turn or lose a mon to sleep and gamble on sleep turns, is a major competitive reason to not exempt Yawn from the sleep move ban.
There is counterplay, of course, but it's really limited:
1) Anything carrying a Toxic Orb (mostly Gliscor) or a Flame Orb (Guts users) is functionally immune to Yawn as they'll already be statused before Drowsy can convert to Sleep.
2) Misty Terrain and Electric Terrain both block sleep, though neither presently has a viable setter.
3) A mon that can underspeed the Yawn user and pivot out would allow a clean entry for a teammate, giving a chance to kill or force out the Yawn user without being put to sleep.
4) A handful of abilities can remove or prevent the sleep status, as can a smaller handful of items.
5) A RestTalk user greatly reduces the penalty of being put to sleep, though it's still a hindrance.
Given that each category only includes a scant handful of options, it really feels like there's just not enough counterplay available to Hazards + Yawn (+ Knock Off) spam, while at the same time such a strategy reduces skill expression (meaning it is, by our definitions, uncompetitive). Even setting aside policy concerns, I see this change making the metagame worse.