Does anyone know of any Arduino projects working with this new protocol?
Getting the performance and security benefits of HTTP/2 for sites and services meant making architectural changes because it upended principles like sharding that had been used to improve web site performance; that may be why only around 35 percent of websites currently use HTTP/2.
HTTP/3 doubles down on that, offering very similar features but replacing TCP with UDP. This time more fundamental changes to networking infrastructure may be required to take advantage of the better performance over poor connections and mobile networks, but for most developers, the change will be transparent.
HTTP/3 is the third major version of the protocol and it includes TLS 1.3 and a new transport protocol called QUIC (Quick UDP Internet Connection); based on a 2013 protocol originally designed by Google, there are now multiple contributors and companies involved through the Internet Engineering Task Force (IETF).
Ref.: https://thenewstack.io/http-3-replaces-tcp-with-udp-to-boost-network-speed-reliability/