Much like in TCP, congestion control really isnt something required for interoperation between peers. Given the userspace nature of QUIC I would expect to see a lot of iteration on this front - for good and bad. (but hopefully the bad iterates quickly).

The current drafts describe newreno is detail, but also explicitly call out the ability to run other things. I've seen reno, cubic, and bbr all run with quic and anticipate others to happen as well. That's one of the exciting things here.