Question says it all. All the Pokemon that have it are pretty bad, except Mega Gengar, which is good.
Shadow Tag has been identified by both tournaments and ladder players as an issue. This ability denies the opposing side one of the key elements in Pokemon; switching. There are a few exceptions to this, such as Ghost types and opposing Shadow Tag Pokemon, but only four Pokemon are remotely viable in OU.
The most popular abuser of the ability is Gothitelle, which gets Trick, Rest, Calm Mind, Thunder Wave, and other viable moves. Gothitelle has the ability to give its item (typically a Choice Scarf) to a Pokemon that cannot kill it, forces it to lock itself into a move, and then proceed to a PP stall until the Pokemon dies by using Struggle. The only way a trap from Shadow Tag can be avoided is continuously double switching until the Shadow Tag user dies to hazards (that means hazards have to be on the Shadow Tag user's side of the field) or giving a Pokemon Shed Shell, which only has one very niche use.
We decided to not suspect Gothitelle because we think Shadow Tag is the main part of the problem. Gothorita has the same trapping abilities as its evolution, and thus could also become an issue should Gothitelle be banned. We also think Wobbuffet and Wynaut, despite being quite unseen and less effective than Gothitelle, are problematic, as they can punish the opponent by trapping a target and create a hole and punish the opponent with little to no counter play.
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Because Gothitelle could very easily shut down and PP stall things. There is no such thing as "good Gothitelle matchups" anymore with Gothitelle being able to pick and choose what it wants to come in against. After that, it's several turns of agony where your defensive Pokemon is Taunted, Gothitelle uses Charm and Confide, and your enemy is killed via Struggle. Shadow Tag removing these defensive Pokemon helped dangerous offensive threats spiral out of control very easily, which is why Shadow Tag recieved the ban hammer.
Shutting down an entire part of competitive Pokemon, which is switching, pretty much means very few/no checks and counters.
