Okay, so if I'm reading this correctly, you are saying that any half-decent mon with STag/Arena Trap is banworthy, and the only reason that Dugtrio isn't a problem is because it's absolute dogshit, correct? Let's examine this argument.
Do the following thought experiment. Imagine we created a mon with sky-high special attack--say, base 170--and gave it an exceptional speed tier--say, base 130, enough to outspeed Arceus himself. Then, we give this mon a pair of powerful STABS that hit the entire metagame neutrally bar Ttar, as well as a defensive typing with multiple resistances and immunities. Then, just for fun, we give this mon an amazing array of coverage moves, to hit basically whatever it wants with SE coverage: Thunder, Energy ball, Icy wind, Dazzling Gleam, Psychic, Focus Blast. In addition, we'll give this mon an amazing array of utility moves--say, will-o-wisp, Toxic, Substitute, Disable, Taunt, and Perish Song. Oh, and just for fun, we'll also give it Destiny Bond so it can trade 1v1 vs anything it can't straight up KO.
Oh, we'll give this mon Shadow Tag, and put it right in the middle of the gen7 metagame just to give it access to Hidden Power as well. Surely this mon must be absolutely awful for the metagame and sent to AG ASAP?
Oh wait, we don't have to do a theorymon about this because this mon already exists. It's called Mega Gengar, and gen7 Ubers thrived with Mega Gengar at the helm.
Needless to say, I don't really buy the argument that anything half-decent with STAG is automatically bad for the metagame.
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There's also been some discussion about Wobb on this thread. I'll just put out my 2 cents on Wobb: This mon has been unchanged since Gen 3, and has not been unhealthy for any metagame due to various flaws it has. Until Wobb causes problems and gains significant non-zero tour usage, I have no reason to believe Wobb is unhealthy for the Ubers metagame, as it has not been a problem for the last 5 generations.
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Let's compare Shadow Tag to an ability that has already been banned: Moody. Moody is broken on literally anything that gets it--even the mighty Arceus trembles in front of Bidoof. Any moody user--even those in LC--has the ability to spam sub and protect to get boosts, and with a little luck it's basically game over. Needless to say, Shadow tag does not meet this threshold. It's only potentially problematic on the Goth evolutionary line, and it has been proven to be fine on Wobb for multiple generations, and wasn't even a problem on Mega Gengar.
Do not ban STAG from Ubers. If Goth gaining Charm and Cosmic Power makes Ubers unenjoyable, ban Goth, and ban Gothorita if you have to, but do not ban Shadow Tag.