Going to pop in and give my opinion. I would be in favor of a Gothitelle/Gothorita suspect. For the sake of brevity I'm going to refer to the pair collectively as "Goth."
The big problem for me when considering Goth is how limited the counterplay against it is. Being able to respond to and counter your opponent's threats and plays in-battle - what we would call player interaction - is a vitally important part of competitive play and is what makes the game interesting. As strong as the other Pokemon in Ubers can be, they for the most part have good counterplay. Using Primal Groudon as an example, as strong and ubiquitous as it is it can be beaten in several ways: chip it, status it, switch to a Pokemon with a better matchup, even trade something with it if the opponent losing Primal Groudon is more important to your gameplan than the Pokemon you traded for it. Even if your team doesn't necessarily have super strong answers to Primal Groudon or they were taken out early, you can still overcome it through smart in-battle decision making.
This isn't the case with Goth due to its always active trapping mechanism. If it manages to come in on the target it is designed to trap, it will win. You can't switch against it or even successfully chip or status it because its targets cannot escape. The only way to prevent Goth from doing its job in a battle is to give it no job to do; ergo, don't use anything it can trap in the teambuilder. Problem is, the list of things it can successfully trap is substantial: most support Arceus formes, Scarf Xerneas, Scarf Yveltal locked into Oblivion Wing (or even potentially Foul Play), Defensive Zygarde-C w/o Dragon Tail, support Necrozma-DM without Solganium Z or SD, Defensive Ho-Oh, most defensive Steels, Blissey, and Toxapex w/o Shed Shell, Lugia w/o Dragon Tail, and Chansey. What all of these Pokemon have in common is that they are defensive in nature or fulfill important defensive roles - Goth's mere existence substantially limits defensive teambuilding options, and because of its lack of counterplay these options are constraining to the point of being compulsory; otherwise, you could be exposing yourself to a dangerous matchup against Goth that you will not be able to play your way around unless you are capable of reading minds and making double switches every time Goth attempts to come in. The incredible limitations on defensive options in the metagame due to the mere threat of Goth's existence ultimately means that teambuilding matchup supplants in-battle decision making when using anything but hyper offense teams, significantly harming the competitiveness of the tier beyond merely limiting diversity in the metagame, and which is why I believe a Goth suspect is warranted.
As an addendum, one thing I would NOT support is banning Goth + Shadow Tag. Smogon tiering philosophy as a whole has considered Pokemon to be self-contained units, and these units generally form the minimum granularity for tiering purposes. Moves and abilities are generally only considered if such a move or ability is busted on all of its users, with OHKO moves and Moody being examples, or if there is a special case that violates an existing clause, as was the case with SleepTrap. Smogon does not "nerf" Pokemon to force them to fit into specific metagames. Banning Shadow Tag specifically on Goth would be no different than banning Dragon Dance on Mega Rayquaza or forcing it to hold a useless item, and you may notice we did neither of that. Banning Goth as a whole thus fits in with our past actions and broader Smogon tiering philosophy and is thus the superior course of action. (It is also worth noting that there would be no practical difference between banning Stag on Goth and just banning Goth.)
One more aside: I personally would not be supposed to suspecting Shadow Tag as a whole. However, it is not immediately clear to me that Shadow Tag as a whole is an issue. Wobbuffet has generally not had a substantial metagame impact, despite being viable and possessing a unique niche. Mega Gengar is more likely to be justifiable, but unlike Gothitelle it does have some counterplay against it through in-battle decision making for two reasons. First, Mega Gengar requires a turn to Mega Evolve. This gives the opponent a chance to react before its trapping comes online, such as by going to a Pursuit user or switching to a strong offensive threat such as Primal Groudon or Yveltal to punish Mega Gengar needing a turn to Mega Evolve and taking some momentum away from its side of the field. Second, while Mega Gengar is capable of confirming revenge kills against many Pokemon, it is not capable of beating many of them one-on-one due to its terrible bulk. This means that Mega Gengar usually requires in-battle setups and maneuvering in order to successfully eliminate targets for which it is reliant on its teammates to make happen, such as creating an opening for it to switch in or chipping a target into KO range. Because of this, one can argue that Mega Gengar is a high skill-ceiling Pokemon that requires some finesse to use to its full potential due to the existence of meaningful counterplay, making it not unlike other strong threats in the metagame. Now, I think this point is potentially disputable, but it is significant enough to where I think it may be better to give Mega Gengar its own debate further down the road. That said, I think its a debate best re-examined after a Goth test, as I believe Goth's warping effect on the metagame is both larger and a more pressing matter to examine.
