Let's not pretent that HO doesn't have the tools it needs to deal with attacking Deo-S. Aegislash and Mega Mawile are two great Pokemon that HO can use and shut down attacking Deo-S. Also, just keeping SR up and forcing it into a scenario where it must use Psycho Boost works, as it's forced out after. Not only this, but HO teams already use a fuckton of priority users, such as T-Wave Thundurus, Mega Mawile, Mega Pinsir, Talonflame, Azumarill, and Bisharp, or some Pokemon which can take one hit from Deo-S and OHKO back, such as Azumarill, Lati@s, and Heatran. So, Deo-S can't switch into anything, it will only KO one Pokemon at best every time it comes in, and will only manage to come in after something is KOed, meaning that the Deo-S player is already behind on the score.
So, HO teams can counter attacking Deo-S if they want (Aegislash and Mega Mawile), can check it (Pokemon that can take its strongest attacks or one hit and OHKO back), and can revenge kill it easily (priority), while giving it almost zero switch-in room. HO teams have all the tools to deal with Deo-S, it's simply a matter of teambuilding if you choose to be weak to it or not.
Finally, even if we assume that the Deo-S kills HO thing is true (which isn't), since when did we ban Pokemon for greatly limiting a playstyle? We only banned Pokemon that had a singificnalty negative presence in the whole metagame (whole metagame, not just one playstyle), Pokemon / elements that made the metagame too luck based, or Pokemon that were obviously overpowered, but never banned Pokemon that greatly hindered a playstyle and forced it to adapt or become way rarer, otherwise we would have banned Kyu-B for fucking up stall in 5th gen, as well as Reuniclus in early BW.