There likely won't be a find steed spell in the game, since i don't think mounts or riding are in it in general.
As for PHB only content, that's what we know so far. PHB plus maybe something more(like with githyanki), but they would have to decide on that, and we don't know what decisions they took.
Also, Larian usually does not overpromise, they promise something, but usually deliver something more. So there is hope, but we have no knowledge at this point. Some people claim they datamined content even up to Tasha's. So we'll see.
