Controller support is 100% confirmed several times through Larian customer support and it will be on right from the start. I'm so happy about this, because I just tried yesterday first time in my life to play DOS 2 with xbox controller and it was pure perfection. I have been all my life a hardcore m+kb old school crpg player and this was the first time of me trying playing crpg with controller and it's way better than I ever imagined.
Mate, DOS2 controller implementation is so good! If BG3 is there or thereabouts, it'll be heaven.