Interviewer: You mentioned you're doing all 12 D&D classes there. Swen: Yeah, we're doing the classes that are in the Player Handbook. They are the ones that are going in. I hope we openly communicated that part because that's what we're doing.
Interviewer: You won't be pulling in any classes, or subclasses, added in new D&D books then? Swen: No, that's why we... The Player Handbook on its own is already quite a lot of work, to do it the way that we want to do it, so we're sticking to that. If we manage to get it all right, we will be very happy, because that's a lot
The older statements seem to have left the topic of including non PHB/MM/DMB material more open. E.g., 2020 Reddit AMA. Swen: "PHB/MM/DMG are the base we are starting from but we are taking material from other books too."
tl;dr: Classes/subclasses are confirmed to be PHB-only. Other non-PHB 5e material like races (besides Githyanki), spells, feats, etc are possible but unlikely given the amount of work Larian still needs to do to get all PHB material in.
