Deo-D was deceptively good at what it did. Thanks to how bulky it was, it didn't need to run Focus Sash which opened the door to a shitload of different item choices like Leftovers, Tanga/Colbur Berry, Mental Herb, Red Card, and even Life Orb, all to get past what might give it trouble in getting hazards up. There are a few Pokemon that already have Spikes + SR, but not only is Deo-D superior in terms of stats (90 Speed is very good given its bulk, and its offenses weren't too bad either as you'll see), they don't even come close to the sheer versatility it had up its sleeve to beat its would-be answers. You've of course got Taunt and Magic Coat to stop opposing hazard setters/Taunt users/setup mons/whatever. Skill Swap also saw some use (at least in BW2) in order to get past Magic Bounce users, and T-Wave, CounterCoat, and Toxic could prolly be used to fuck over sweepers or walls. And then of course, because Deo-D is still a Deoxys forme, it's got a great offensive movepool to choose from if needbe. 70 Atk/SpA isn't enough for a consistent offensive presence, yes, but it IS enough to let it break past specific mons as its team needed. It's got stuff like Thunder/Thunderbolt, Psycho Boost, Superpower, HP Fire, Ice Beam, Knock Off, and so on; it could beat so many offensive threats given the right moveset and item choice (I wouldn't be surprised if people would try running Z Moves on it if it was legal rn). While obviously it can't do all of these things at once, it could just be tuned to whatever your teams needs, so its ability to get up hazards on top of how many ways it could snuff out an opposition made it banworthy.