Alright after god knows how long, I'll make my contribution in this discussion or whatnot. I'm going to ignore the aspect of shadow tag and focus primarily on gengarite since MM2 will give me a beating if I talk about shadow tag rip.
From what I can see, the arguments in no particular order of importance.
Pro ban side:
1. Shadow tag is broken and we need gengarite banned to ban gothitelle
Let's be honest now, unlike other suspect tests in various tiers where slippery slopes are to be avoided like the plague, this particular suspect test is an exception to that since we know for a fact that if gengarite gets banned then a suspect test for shadow tag as a whole soon follows as quoted by fireburn
here. So although some might feel that gengarite isn't broken necessarily but rather goth is broken, must be forced to ban gengarite in order to achieve said goth suspect. This is up to general preference and has no bearing on whether or not gengarite is broken but this fact will sway voters decisions so eh. I feel that if anything we should either have several voting options such as; ban gengarite, ban gothitelle, ban gengarite + gothitelle, ban shadow tag, ban wobbuffet, etc. Doing this would probably allow us to have a more constructive suspect discussion (not like they're ever constructive in the first place) and allow us to find out the root of the problem rather than this back and forth we keep having.
2. restricts teambuilding by making grass/fairies near impossible to use (mostly grass/fairyceus + slyveon) but also forces us to not use any mons that is potential bait for gengar (chansey, and other walls mostly that have next to no offensive presence)
In all tiers, team building is restricted to an extent due to the vast majority of threats that must be accounted for however in this particular instance, it "removes" choice by disallowing us from running gengar bait mons so to speak. How is this different than other major threats that rather than forcing you to not use x mon, they instead force you to use y mon to deal with them. Example given, you're not allowed to run mons such as fairyceus/chansey because of gengar's mere existence or even if you opt to run those mons, you have to gimp them in order to deal with gengar. How is this different than being forced to run checks to a certain threat? Such as when teambuilding you absolutely have to have kyogre/xerneas/blaziken checks otherwise you will be at an incredible disadvantage when faced against those mons. They're essentially the same in essence since they both "remove" choice from teambuilding. If gengar is is so dominant to an extent that it makes several archetypes unviable then rather than try to use playstyles that aren't effective, wouldn't it be more preferable to adapt to the metagame and run the mons/sets that are viable and instead stick to those archetypes with their respective sub categories?
3. In addition to the above point, it makes team matchup that much more important.
melee mewtwo touched upon this earlier by mentioning that shadow tag makes team matchup more important but with or without shadow tag, team matchup will prevail so this is mostly up to preference since a good team should be able to at least check the vast majority of threats in the tier while still being able to fulfill their win conditions. I don't feel that gengar can break down a team by itself but I feel it does make each and every move more important but that's part of the skill involved in the game so no big deal there.
4. Overpowered/overcentralization that is unhealthy for ubers
As far as being overcentralized is concerned, that is essentially irrelevant as far as the uber tier is concerned. This isn't ou or some other tier where overcentralization should be avoided at all costs, this is fucking ubers with mons that constantly see usage of 40%+ (kyogre since like forever, xerneas since xy started, etc) and we never tried to push for a ban on those for overcentralization since their usage stats are by far superior to gengars.
The other concern is being overpowered, and with its combination of shadow tag, high speed and high special attack, it's hard to see how this wouldn't be overpowered with the ability to remove key mons on your opponent's side however in order for it to do so, it has to first mega evolve which is quite hard to do in a game due to gengar's relatively low bulk and inability to switch in on mons since even most neutral hits are solid 2hko's. If it however does manage to come in and mega evolve and manages to remove multiple threats, wouldn't it be due to the fact that said mega gengar user has outplayed their opponent which we encourage in the "skill" based game of pokemon and our tiering goals should be to ensure that the better player will most likely win and gengar is just a tool to achieve that.
5. Removes choice from the player
This is something I feel the proban side brings up alot, for reasons that are beyond me. Adding on to what conflict said earlier, unlike gothitelle, gengar has a far harder time being able to set up (get its shadow tag essentially) and take out a key mon. The only thing I can really agree with on this side of the argument is that it forces a shit ton of 50-50's between taunt and destiny bond which is a high risk high reward play. Though would these 50-50's be considered broken to the extent that it warrants a gengarite ban? Unlike gothitelle, who essentially autowins provided it comes in on the right mon, gengar is forced to risk itself to ko said mon which is far from reliable.
Anti ban side:
1. It's wrong to ban a mon from ubers WTFBBQFUCKSMOGON
Smogon has shown before that we're willing to ban strategies and/or mons that are noncompetitive in the ubers metagame (think arceus in BW ubers where locopoke and other esteemed users tried to keep it banned) and we've also removed evasion clause and banned swagger which was "against" our philosophy so it's not really unusual nor wrong for us to ban shadow tag should the users deem it as uncompetitive.
2. Can be pursuit trapped
Thanks to how shitty most pursuit users are, it's not exactly easy to switch a pursuit user in since thanks to gengar's high SpA and plethora of moves which include hp fire, shadow ball, focus blast all of which are relatively common, no viable pursuit user can safely come in and if a pursuit user comes in after gengar has ko'd something then gengar has already done it's job.
probably missed some irrelevant shit but i assume they're not important since I didn't see/remember them but if I remember I'll probably respond to them in turn.