Short answer, Baxcalibur was already considered as broken before the DLC. Then the DLC came and gave it fking scale shot and Alolan Ninetales returned. Under veil, FrostZilla is nearly impervious to anything especially physical hits due to how hail works now where it increases ice types physical defense. Combine that with sd scale shot and icicle spear with loaded dice, this thing tears apart entire teams. I should know because I spammed the sht out of this when it dropped along with good ol scale shot Garchomp. Having said all that, people were screaming for a ban in the OU forums then not even 24 hours ago from when this answer is posted, the first survey for the DLC was dropped and about five or six hours earlier, FrostZilla was banned due to overwhelming support from both the community and the council
You can read the official article here
Baxcalibur was already one of the most potent Pokemon in the metagame and perhaps on track for a traditional suspect and ban if the last metagame had more time. However, DLC came and many people thought this would bring increased counterplay to Baxcalibur with more fast Pokemon, more Fairy types, and so on. With the addition of Scale Shot, the set mix of Baxcalibur became more threatening as it was able to boost its speed while doing great deals of damage. People already used Loaded Dice with Icicle Spear, but now there was even more reason to use this. Throw in the conventional sets, the likelihood of support from Snow Warning and Aurora Veil, and the Terastallization component that allows it to circumvent common revenge killing options and you get one of the strongest options SV OU has ever seen.
We elected to include Baxcalibur in the first post-DLC SV OU tiering survey, which is still ongoing as we speak. While I do not have a formal final score for it yet, I can say with confidence that Baxcalibur's score was one of the highest in the history of our tiering surveys. This prompted us to conduct an emergency council vote, which is similar to how we handled Regieleki with the release of Pokemon HOME.
