I am pro-ban, but I do just want to make a point about Shed Shell and one of the poorer "pro-ban" arguments.
Forcing Pokemon into specific item choices or move choices is nothing new and is not indicative a particular threat being broken. Rather, it is completely inevitable as a metagame adapts. There are many, many examples of Pokemon using highly specialised sets that really only beat 1-2 Pokemon at the expense of others, but that doesn't mean those threats are broken, it just means they are worth preparing for. You have to take into account what is currently good when you teambuild, whether that means running an overly specially defensive spread on a wall to beat Kyurem at the expense of a physical threat, or an obscure coverage move to help wall-break, or Toxic in an unoptimal place, etc etc. Bear in mind BW Keldeo briefly went through a phase of running Expert Belt + HP Bug exclusively to beat Celebi, that DPP Heatran often uses Passho Berry + HP Grass/Explosion to beat otherwise scary offensive waters - these highly specialised sets are a sign of innovation to flip a match-up on its head, not of oppression.
So the issue here isn't that you are pushed into using Shed Shell. The issue is that Dugtrio can support a hugely diverse range of partners which all enjoy the removal of different Pokemon, and therefore Shed Shell in one slot actually doesn't Dugtrio-proof you in the same way as the previous examples. All-in-all, Dugtrio just gives too much influence to team match-up - did you allocate your very limited teambuilding resources to beating this niche Dugtrio strat, or did you miss it in favour of a different set-up?
Sure, when you teambuild, you have in mind combinations like Dugtrio + offensive dragon that want to remove your fairies, so you might be tempted to use Shed Shell Clefable. However, this does nothing for you against Dugtrio + Venusaur on sun, which gives 0 fucks about Clefable and instead wants to remove your Tyranitar / Kyurem / Toxapex, etc. As a result, you have to factor Dugtrio into your teambuilding repeatedly if you actually want to be adequately prepared for all the permutations of Arena Trap support.
At the end of the day, there are so many possible set-ups using Dugtrio that it becomes overwhelming to prepare for every variation. If it was simply a case of slapping Shed Shell on something then that'd be fine, but in reality you've only protected yourself from one of the Dugtrio strategies. If you have anything grounded, you are still at risk of being cheesed by a niche set-up that specifically wants that threat removed, and unfortunately in many cases, when the trap is off then its a near auto-lose. These kind of situations, where one team has a highly-specialised set-up that gives an overwhelming match-up against specific archtypes in the metagame, is one that ruins tournament competitiveness and I think most of us would rather see gone.
(And this isn't even mentioning Knock Off, Trick, Gravity and the myriad of other weird and wonderful ways that hyper-specialised Dugtrio teams can find ways to bypass your preparedness for Arena Trap.)