TL;DR, Summer 2020: Development of QUIC and HTTP/3 within the IETF has reached a significant milestone as it nears completion. Most implementations are able to interoperate with each other and servers are ready to serve QUIC and HTTP/3. And, while clients are at various stages, client support is starting to roll out as well.

https://www.fastly.com/blog/state-of-quic-and-http3-2020