Why is sleep broken? It doesn't matter if it is broken.
The real point is that it is uncompetitive. Let's compare it to 3 things that are banned without even need of suspect: OHKO, accuracy lowering, evasion.
1. OHKO - Let's compare the math to the least accurate sleep move that is utilized, Hypnosis. Hypnosis has 60% accuracy with 66% chance the sleeping mon does not wake up first turn. .6*.66 is about 40% chance you get the free turn to set up the win. Obviously the math gets better as you use higher accuracy moves. OHKO moves are 30% accurate and you don't have to option to set up a sub to protect yourself from the failure that is first turn wake.
Additionally, the potential "abusers" of OHKO would be rather pathetic.
Sheer Cold: Abomasnow (probably the most effective abuser of all OHKO);
Guillotine: Kartana (I guess it doesn't gain too much from coverage), Pinsir (has much better things to do), Durant (going swarm?);
Horn Drill: Excadrill (niche move for niche mon, at best), Dragonite (did you forget your movepool?);
Fissure: Zard (why?), Donphan/Golem (maybe you finally beat Gyarados! if he doesn't flinch you...), Lando (why have nice things?)
Counters to OHKO may be far more numerous than counters to sleep. Sturdy has more users than Magic Bounce, more mons in the meta are immune to Fissure or Guillotine/Horn Drill than immune to sleep powder, and the counters to the actual pokemon that would use OHKO moves are numerous.
2. Accuracy lowering - The math on this is also clear. If Venusaur uses Mudslap, the opposing Mawile has a 75% chance to land an attack and a 100% chance to set up a Substitute or Swords Dance next turn. Meanwhile, if Venusaur uses Sleep Powder, Mawile has about a 50% chance to do anything at all next turn (75% accuracy * 66% you don't wake up). Additionally, accuracy lowering has 0 impact on landing a Z-attack or receiving Z-status boosts, while Z-moves cannot be used at all while asleep.
Potential abusers of accuracy lowering include: Venusaur, Chansey? Both of these have much better move options. Am I missing something else that would bother?
Counters include Clear Body, substitute/ground immunity for mud slap, taunt & magic bounce for flash.
3. Evasion - Evasion is a stronger mechanic than accuracy lowering (not the point of this discussion) but the math is effectively the same as for accuracy lowering. I'm out of steam so see math above.
Replies to previous comments:
Serene Grace should be banned too (discuss soon after). Paralysis has different math so that's a different discussion.
Greninja & Koko beat Snorlax or Venusaur? There are 2 Magic Bouncers. How many usable grass types are there? (just the ones that can abuse sleep?). Put Crunch on Snorlax sometime and enjoy a day of freelo. Substitute - Smeargle has quick claw for a reason, good luck subbing on Gengar, Jumpluff.
In summary,
no sleep suspect. Quick ban as you did with accuracy lowering.
- how many likes does it take to hold up a planet of text?