I think we have users who have written proxies using Helidon 4 (based on questions we’ve previously received) but these were specifically built to handle specific needs.

However, Helidon does not yet have some of the features you would expect in a general reverse proxy e.g. load balancing, URL re-writing, caching etc.

If you want to build one on top of Helidon, then Helidon's use of Virtual threads and the blocking APIs fits this use case well and that would make Helidon a reasonable choice upon which to build it. If you're looking for an out-of-the-box reverse proxy, then something like Apache WebServer or nginx is a better choice.