Well... my Xurkitree set can sweep and win 6-0 pretty easily.
Zap Cannon + No Guard should be banned too.
Really, Diancite beats the heck out of Xurkitree.
Sleep Clause means that you can't use a sleep inducing move while the opponent is asleep (ONE SLEEPING OPPONENT AT A TIME).
6-0 doesn't mean anything. If the opponent has not a single Mega and you 6-0 them it's still a 6-0, just against an unskilled player. No Guard Zap Cannon can be walled pretty easily by Blissey and even Specially Defensive Magearna or Dusk Mane Necrozma. Diancite doesn't necessarily beat Xurkitree; it just allows the holder and incredible ability to revenge kill with high offenses and speed. With enough power a lot of things beat Xurkitree easily. Xurk isn't exactly what you'd call bulky.
Sleep Clause means that no more than one Pokemon in the party can be put to sleep
Also I wrote this so lol.
It isn't enough to lessen the disgusting brokenness of sleep spamming,
Sleep Clause is exactly what keeps it from being "disgustingly broken". With it you can "sleep sack" a non-check and go into a check without fear of being put to sleep. You find switching and letting another Pokemon take damage unfair? Then literally don't play competitive Pokemon lol. Magic Bounce stones are far from sub-optimal or highly situational; in fact they make great stones both defensively and offensively. You never elaborated on the "switching and letting another Pokemon take damage" part enough. Switching is one of, if not the most important components of competitive Pokemon itself; it allows people to switch out their Pokemon in unfavourable situations into something that can deal with said Pokemon on the opposing side of the field. If you don't understand that concept, I honestly do not think that you have authority to rant about how something so easily countered can be broken in any way possible. Let's look at the possible checks to the so-called "broken" combos:
Prankster Spore (Breloom @ Banettite)
-literally the most used Pokemon in the tier, Altarianite Zygarde
-other relevant Flying or Fairy types such as Pinsirite/Salamencite/Altarianite Landorus-Therian
-fast revenge killers that hit hard on Breloom, e.g. Glalitite Weavile
-literally all -atespeeds
No Guard Hypnosis (Gengar/Xurkitree @ Pidgeotite) (I'm sorry, Alolan Raichu has never been and will never be relevant due to its frailness and susceptibility to priority and strong hits in general)
-Thousand Arrows on any variant of Zygarde
-a Primal Groudon that is not asleep
-something that can stomach special hits in general and possess Magic Bounce like Sablenite Blissey or Magearna
-literally anything with good offenses and speed exceeding base 130
-once again, strong priority
These aforementioned checks are something you and I can both come up with; yet you argue that it can break teams by allowing a Pokemon to be immobilized and subsequently die and have this cycle endlessly repeat until all 6 mons are down. That is a simple newbie mistake that most experienced players, even ones not familiar with Mix and Mega (but know of the existence of Sleep Clause and sleep sacking) know not to make, and those who do in fact lose to said "strategy" are simply unskilled and do not understand the fundamental skill of utilizing Sleep Clause to their advantage.
As previously mentioned, I don't understand how switching and letting another Pokemon take damage is detrimental to the general game and therefore makes "sleep spam" broken. If anything, it makes sleep spam far less broken, due to there being no possibility of immobilizing another Pokemon and rendering it useless.
Sorry all of you experienced players have to read this, I just feel that explaining this was necessary.