I've always been in the camp that all Pokemon should be catchable, that they are all just creatures of varying sentience and capability.
Flavor text like with the DPPt box-art legendaries are myths believed by people who witnessed particularly powerful Pokemon in ages past, before technology/trainer culture developed to a point where such creatures could be, or would be, caught.
And by the time human civilization does advance enough, it's so ingrained into the local culture that few, if any, would dare try outside of the exceptionally skilled (you) or particularly diabolical (the Teams). And at first, the Pokedex would record the creature in its mythological context, because not enough scientific data would be yet known.
To put it more simply, there is a difference between godly Pokemon and Pokemon gods, and we have only encountered the former.
And to bring it back on topic, I think it's the same for the Ultra Beasts. Being seen as removed is not necessarily the same as actually being removed. There's every chance that the Ultra Beasts are catchable.