I need one to complete my Pokedex. I know there were events in the past, but I missed out on those. Is there any way to somehow claim one still, in some sort of DNS exploit-esque fashion? I can also use Y and Omega Ruby if there's some way to get one there.
I'm pretty sure I know the answer already but I'm still holding out hope that I'm wrong.
No. Because Game Freak is no longer making events for USUM also there was never a Volcanion event for USUM.
However there is a SwSh event called "Get Challenge Volcanion" ocurring now, you can get it online in any region and expires in April 30th.
Hope I helped.