I severely doubt it or Xanathar's will be in at launch. They have so much to develop already. More subclasses wouldn't be hard to integrate into the game post-launch considering the story is most likely reacting to classes, not subclasses. So, adding subclasses would solely be affecting character specialization systems and not the story cohesion of the game world as it relates to the character. It seems a no-brainier from a development perspective to focus on core functionality, story, etc while supporting the vanilla PHB class mechanics for now, then providing more bells and whistles for character specialization in an update. That way, players can enjoy a complete experience for their first playthrough(s), then have more replayability options for future playthroughs.
At least, I feel like that's the smart approach. I think all the development resources that can be put on things that enrich and deepen the story and reactivity of the game to your character should be high priority, because it's harder to iterate on the story post-launch. Also, the combat needs to feel fun and representative of a true D&D 5e experience. Figuring out ways to ensure the 5e systems transition well to combat in the game are of paramount importance, because changing the combat systems and player abilities after launch is more jarring to players than adding more subclasses. For instance, right now it is probably still the case that the systems governing how Reactions work need heavy iteration.
Bottom line, I honestly feel like there are too many more important things to do and perfect to focus on a breadth of content from many supplemental books. Implementing the many features, spells, abilities, and rules of 5e into a video game is not a small task if you want it to be true to the tabletop experience. Few games even try. The 5e PHB, MM, and DMG kept players busy for a long time at the table, and I'm sure it will do the same in BG3.
Do we know the game won't react to subclass, I mean they haven't mentioned it one way or the other yet as far as I know.
