Thanks for your interest. Netty is a great networking framework, based on my experience.
All deployments of QUIC I'm aware of are currently based on Google QUIC, typically supporting v35 and above.
Google is actively working on migrating towards the IETF document you linked to, but we're months from having implemented it as it's currently specified.
If your hope is to interop with QUIC deployments at Google, Chrome, and QuicGo(
https://github.com/lucas-clemente/quic-go) in the next few months, you'd want to implement the Google QUIC. If you're willing to wait longer, then the IETF document you pointed to is the right place to start.