Do you mean libraries for backend languages, like Go and Java, or servers, like Nginx and Caddy? Because for Go there is the quic-go module[1], and since Caddy is written in Go, it already has (experimental) support for HTTP/3[2].

[1]: https://github.com/lucas-clemente/quic-go

[2]: https://caddyserver.com/docs/caddyfile/options#protocol