None, the web has always been (or at least intended to be) backwards-compatible, if you disable concatenation (for ex.), your HTTP1 users will suffer.
OTOH, techniques like ES6 modules won't take advantage of HTTP2 if you implement them incorrectly, there is a reason why they aren't supported by browsers yet.
I think the question was meant to ask what new features are implemented, that will render HTTP1 conventions and methods useless, even if they might still be compatible.
Vijay Thirugnanam
Inference Service @ Cerebras
If server push becomes mainstream, calling web API to retrieve data becomes redundant. Web will be modelled similar to socket.io and Meteor publish / subscribe.