I probably don't have much to add to what has already been said but here are my two cents based on what I've experienced in the ladder:
I personally didn't find SpeedPass to be actually broken. Annoying? Yes, but definitely not broken. In itself SpeedPass is a kind of a shaky strategy (imo) in view of how frail and/or easily revenge killed, be it by priority or something holding a Focus Sash, some bp final recipients really are. But at the same time, it puts an insanely high amount of pressure on the opposing team since they absolute cannot let it happen safely. But, isn't it the same for any other strat? The only difference I see is that this one can be accomplished in no more than 3 turns if everything goes smoothly—it happens all too fast. Choke, even if just once, and you're done. The tier doesn't have a blank answer to it.
Anyways, surprisingly I think I've faced no more than a couple of teams using Chicken Pass, and only lost to it once. Ninjask was, overall, far more common which is funny because Combusken can actually scare things off on its own so I thought I'd see it more often, but that's neither here nor there. At any case, as worrysome as this cheap trick is, I don't think it is limiting teambuilding so much as a few moves here or there, so as of now I am inclined to vote against a ban. I'll play a few more games to make up my mind before the voting thread goes up
yea its broken lol
So near the end of my reqs campaign, I wanted to try out chickenpass, not expecting much. I had dappled with regular bp before but not much had really come of it, as priority or good prediction usually stopped it in its tracks. I was astounded at how easily it broke opposing teams. Most other speedpassing mons, like ninjask, dont have good bulk, wisp, good type synergy w/ top receivers, and/or are hazard weak. Comparing Combusken to regular speedpass is fallacious in the sense that combusken has tools that other speedpass users would die for, and it can already utilize what it's given very well (i.e, wisp, evio bulk, syergy w/ receivers, etc.). You mention sash users; the only really relevant sash user is jynx, and if you just protect and pass to something like malamar, you're fine. There is no definitive breaker of chickenpass (aside from crap like aggron, which isn't taking a wisp too well.)
Also, I see a lot of people sayng "well, I don't see it a lot".
JUST BECAUSE IT IS NOT SEEN ON EVERY TEAM DOES NOT MEAN IT ISN'T BROKEN. I can chickenpass out to CM xatu, get up 2 CMs, get 2 kills, and be forced to switch out. There are so many oppurtunities for chickenpass to happen, and in my view, this is what makes it really broken. You have multiple oppurtunities to break the opposition. After I take out Xatu, say, from Piloswine's Ice Shard, I can just go out to something else, and resume regular play. Then, when the time is right, I can just go back into chicken and repeat the process. Chickenpass teams are perfectly capable of operating normally without boosts, as well. This makes it even harder to play againt, as this means you're never really able to neuter the strategy.
Lastly, NU is a slow tier. The fastest relevant mon would be something like a scarf scyther or primeape, which are both easily checked in the cycle of chickenpass, thorugh WoW, Xatu, etc. The versatility that it brings to the table is tremendous in this sense. Facing a scyther? Pass out to SD rhydon and sweep. Facing a kabutops? Pass out to Samurott and sweep. There is no shortage of options here. Also, any sane person accounts for chickenpass when building. It's pretty much mandatory to have ways to deal with it, however futile these ways may be.