One thing: Deoxys-D is not the core of weatherless teams, it's the core of HyperOffensive (HO) teams, usually, HyperOffense is did on weatherless teams, but that's far from being all the picture, as an HO team can be used at the same time of other playstyle without any major trouble.
Anyway, I find Ferrothorn a lot more broken than Deoxys, Ferro walls the 90-95% of the metagame and can easily setup SR + 1 layer of spikes + Leech Seed something, it's unique trully weakness; fire, is easy to cover, and yeah, that's pretty much all, Ferro setups hazards on the face of almost the entire metagame, and still walls and annoy the entire metagame afterwards, while Deoxys-D's unique job is to setup SR + 1 layer of spikes (2 layers against newbies or getting lucky), and that's pretty much all, generally.
Of course, Deoxys-D is somewhat useless against teams prepared for SR, or with spinners, that are almost... everything on the tier, yeah, he's not that horrible to handle, he's just a hazard spammer, and is somewhat useless outside of it. Proof of that is the fact that HO teams are really far to being broken, they are threats, but they are really far from being broken, and even if we ban Deoxys-D, nothing will change, as Forrestress and Ferrothorn does the same and usually better.
Deoxy-D's edge over the other hazard Pokémon is the fact that he's not weak to Ninetales or Hydreigon leads, that's a nice edge, but usually, that problem can still be handled with other Pokémon and just leaving the hazards stage for some turns later.
Still, Ferrothorn and Deoxy-D's presense makes you a must to have a counter to them and of preference, a spinner, as otherwise you're on a big, big trouble, and that restrict your options at some degree, so, I could rather be OK by banning Deoxy-D's as long Ferrothorn is also banned, otherwise doesn't have too many sense. This suspect doesn't have too many sense, anyway, HO playstyle is rarely a top threat, and just banning or not Deoxys-D you're not stopping the usual spam of hazards on the tier, anyway.