In DOS2's case I guess the PvP tag was added because it had a separate game mode called Arena where players could fight short battles against each other using a selection of preset characters. The main game also had some optional PvP elements, but I'm not sure if their presence is enough to warrant the addition of a PvP tag to the game's store page.
I assume BG3 will have something similar to DOS2's arena game mode. I don't know how popular that game mode actually was, but I imagine it'll be a good way for people unfamiliar with the 5e classes to try them out without having to play and level them up in the game's story campaign.
