Hello!
Before I begin this post, I want to say that I am fully supporting the banning of this Pokemon. I've achieved suspect requirements and this is what I've decided I would like to vote.
However, it seems the discussion here has been sort of sprawling in all kinds of directions, so I want to bring it back up to pace.
See, the fact that everyone is so unified on Pheromosa being banned is nice, but also kind of weird imo, because there are some valid arguments for why Pheromosa might be able to stay. So in this post I sort of wanted to go over Pheromosa in detail and highlight its pros and cons a bit, maybe help get some good discussion going.
So with Pheromosa, the first thing to note is that it isn't as limiting to teambuilding as one would originally think. TDK outlines how priority is a good way to take down Pheromosa, although Pheromosa is bulky enough to live some priority in its common forms, and even says that Quiver Dance Pheromosa can escape the KO from Water Shuriken, a means of priority which otherwise would beat Pheromosa. The first thing to note on this is that people aren't running Water Shuriken on Ash Greninja only for Pheromosa -- being a reliable check to most Pheromosa sets is a boon for sure, but Ash Greninja needs that powerful priority options anyway to threaten the heavily-offensive metagame that SM has become, so it isn't exactly limiting in this regard. It is actually a highly-used mon that can check Pheromosa very well.
A lot of new metagame trends such as Banded Zygarde and DD Zygarde, both which have a very powerful priority attack, and the general rise of Gengar and even Mimikyu's presence are all very good ways to shut down Pheromosa. Some players speculate that Gengar's rise in usage is only because of Pheromosa, but I really feel like it would be more accurate to say that the majority of the SM OU playerbase simply wrote off Gengar is bad because of its loss of levitate, and as the tier developed people realized that Ghost/Poison offensive typing with a strong Fighting coverage move will always be good in any OU Metagame, and that is why it rose. Even with Pheromosa's absence, Gengar's overall viability won't be affected. Further more, Band Zygarde hits a lot of balance hard and is one of the strongest Priority Mons in the tier; priority cores (Zygarde Band + Mega Maw being the most common priority core) can blanket check more threats than simply Pheromosa, and will still be strong even with its absence.
Toxapex isn't a super limiting Pokemon to run because it is one of the tiers best blanket checks to mostly everything; Pheromosa is just one other Pokemon Toxapex can beat amongst the many it can counter/check for its team.
Contrary to what this thread has implied, Speed-boosting Pheromosa sets aren't particularly common or even Pheromosa's best set imo, as the Pokemon is plenty fast either with a Scarf or its neutral invested speed / QD boost as aside from Scarf Pokemon it really doesn't miss much. With this said, Choice Scarf Pokemon are used for a lot of things in the metagame, Zard X and Volcarona and Gyarados/Salamence being the users that come to the top of my head. For these reasons, Priority / Choice Scarf Pokemon / Webs / any form of speed control is really valuable in this metagame and can pressure / check a lot of Pheromosa sets as Scarf is one of its lesser-used sets as are the Speed-boosting ones. Not saying you can 100% rely on Scarf / Webs / DD / QD to jump Mosa, but they are things that a lot of teams run that can check a majority of Pheromosa's best sets. One of the best Pokemon in the OU tier, Mega Metagross, has one of the strongest priority moves in Bullet Punch and is bulky enough to live most attacks Pheromosa can throw (Focus Blast Specs can OHKO and Z-Focus Blast has a roll), making it a very sturdy Pheromosa check.
Counterplay involving Pheromosa highly depends on its set, but it isn't as hard to scout as some players make it out to be. All special attacking Pheromosa sets (QD, Specs, LO Special) are all handled through things like Toxapex, Gengar, Marowak, Tapu Fini, and priority/scarf anyway. The thing with special attacking Mosa is that they don't normally carry U-Turn or Poison Jab, so they usually don't come out until lategame as they cannot spam U-Turn to get out of bad situations. The physical set will find ways to come in earlier in the game than special attacking Mosa and the moment you see U-Turn you can pretty much always beat Mosa with checks like Venusaur Landorus-T Marowak Gengar Toxapex Intimidate Gyarados Zapdos etc. that are highly splashable on teams; in fact, physical mosa has a lot of common checks imo that make that set seem underwhelming. Physical Speed Boosting nature Pheromosa was the only set in early SM OU and it was so underwhelming that it didn't even make S Rank and saved it from a quickban imo. In terms of scouting Mosa it is almost like Charizard scouting in the sense that while you don't really know which mon / spectrum it is going to hit from, the way both sets play are so different as well as the teams they are put on that it can sort of clue you into the set they have.
Ghost types are good since there aren't many resists in the tier atm, priority is strong in a offensive-based metagame, scarfers are good in the speed-control meta, all fairly good checks to mosa sets taht aren't going to go away anytime soon.
People get held up on the fact that Mosa doesn't have many "counters" but a lot of things don't have "counters" anymore and especially offensive teams you're mostly relying on checks anyway. Offensive teams especially are check-oriented, and a lot of checks are splashable that can counterplay Mosa.
-.-.-
However, despite these things, I feel Pheromosa should still be banned. While I do not think Pheromosa is as problematic as Mega Metagross, Pheromosa is versatile and still fairly unhealthy to teambuild around. It has huge offensive stats and access to a fast + powerful U-Turn which allow it to dent a lot of its checks/counters and it has the ability to use Z-Anything to blast through most of its counters. Z Poison Jab removes Tapu Fini, Z Hyper Beam can remove Toxapex (and also Tapu Fini), Z Focus removes the need to hit Focus + lets it damage through resists like Zapdos or what-have-you.
It is naturally faster than most of the metagame, and giving it a Choice Scarf will ensure it outspeeds everything (even Pokemon like Kingdra and Excadrill that teams spend several teamslots to support).
Its bulk may appear bad, but it actually enough to live a lot of random priority attacks such as Bullet Punch from defensive Scizor and stuff, making it feel more bulky than it should be from 100%, so priority teams need to rack up hazards to put it into KO range or other means of chip damage, which can be frustarting / limiting at times.
The most intimidating part about Pheromosa is its ability to hit from either the physical or special spectrum with incredible power and access to one of the most powerful boosting moves in the game. It has item freedom, even things like Focus Sash on Pheromosa isn't bad and lets it shutdown would be forms of RK assuming you keep rocks off. It also gets Rapid Spin, which is very good as offensive teams are really hard pressed to find hazard control on their teams with only Mega Scizor Defog, Excadril (Scarf or Sash) and Tapu Fini as ways to remove them.
Pheromosa can hit from any spectrum, forcing players to respect it and sac several Pokemon just to identify its set. You can argue that Metagross Mega does the same thing, but I would argue that Metagross is also ban-worthy so not sure if that argument holds.
-.-.-
While I will be voting Ban on Pheromosa, I am a bit shocked to see that there aren't more people that wouldn't want it banned. It has reasonable counterplay measures, and things like Mega Metagross and Protean Grneinja are arguably harder to prepare for. If there is one thing I'd like to say, it is that even though Metagross and Greninja probably deserves a ban and might have been better canidates to ban first, it doens't actually matter when something is suspected so long as the right choice is made. Pheromosa should be banned, especially in metagames without Metagross or Greninja imo, so it is the right choice to ban it.
I know it is sending mixed signals to offer so much anti-ban arguments, but a lot of people were asking for some so I thought I'd share my reasons why I felt Pheromosa was a bit more grounded, even though I personally think it is broken and ban-worthy.
Happy laddering! Can't wait to ban this thing and move on to Mega Metagross.