Comments on Shadow Tag: it's a terror if you misplay or if you don't have hard counters but the Encore-Perish-Trapper isn't the only thing it can pull off: you can use it as a member of a Baton Pass chain while the opponent is normalized or crippled by any other means. It just spam Acupressure, then it Substitutes, then it passes everything to a Dark type and this last one goes to town sweeping everything with +6 860bp STAB Power Trip, and it has room to Imprison Haze+Dbond (also the dark type might run whatever ability it wants). For example M-dos OHKOes everything bar Shedinja. And you can just run Mold Breaker.
Even by trying to argument that "Shadow Tag Gengar has checks and counters" then I'm sorry but you are gonna have a bad time; you can pull off a strategy that removes conventional Pokemons that usually check or counter it. That Deoxys I posted some replies ago is an example, but other exist. You can run Reflect Type to avoid the 3HKO from any viable Revelation Dance user.
252+ SpA Yveltal Revelation Dance vs. 252 HP / 252 SpD (Dark/Flying) Gengar-Mega: 66-78 (20.3 - 24%) -- guaranteed 5HKO
252+ SpA Kyogre-Primal Revelation Dance vs. 252 HP / 252 SpD (Water) Gengar-Mega: 84-99 (25.9 - 30.5%) -- guaranteed 4HKO
You can run Baton Pass to escape a Pursuit Trapper. You can use Guard Split to steal the bulk from a wall (obtaining a bulk of 60/115/122 from a Registeel which isn't bad at all with max EV). Taunt to stop Parting Shot/Baton Pass. Protect+Disable to avoid a Core Enforcer/U-turn/whatever. Soak on Ghosts to trap them. The only fair point it has is that you can run it only once.
Comments on Magnet Pull: more specific yet more general. Being its primal user Groudon-Primal (pun intended), you can generally deal with that due to Flash Fire, Primordial Sea, Prankster, being Aegislash, being faster than it (it's just Kartana, Solgaleo and Metagross-Mega), Shed Shell, King Shield and exploiting its crippling weakness to Water Shuriken.
But Groudon-Primal isn't the only viable user. E4 Flint used it effectively on Reshiram
here. Mewtwo-Mega-X uses it just as good as Groudon since MMX totally can run Adamant due to 130 base Speed, still has SE STAB on Steels, and he can really just stick with V-create with results comparable to Groudon-Primal.
Conclusions: Shadow Tag needs to be banned. We just started to scratch its possibilities and counter play to today's standards will develop. It will come a day where we'll be forced to prepare to Gengar or prepare to the meta.
Magnet Pull: It works on 20% of the tier. Sometimes. Stakeout and Tinted Lens bend the meta much more than Magnet Pull. Do not ban, and not even worth the suspect to me.