Recently UDP port support was added to Load Balancers (see also e.g. https://docs.digitalocean.com/products/kubernetes/how-to/configure-load-balancers/#protocol). This allows for various useful applications, in particular support for HTTP/3.

However, this protocol does not allow using PROXY Protocol as of yet (as has been confirmed by my contact with DO support). This means that it is not possible to use the client IP for any purpose when using both HTTP/3 and TLS Passthrough on a Load Balancer.

This idea suggests implementing the PROXY Protocol for UDP traffic as well, so that the original client IP is exposed to the service behind the Load Balancer.