Wouldn't that be a regular proxy, not a reverse proxy?

Yes, but only by techie-social convention. Outside of web servers, the party hidden by the proxy is conveyed in context.

I think it’s pretty relevant actually? You can’t use HAProxy, it won’t work, because it’s only a reverse proxy. Squid will work.

I wasn't saying it's irrelevant, I was saying that stipulating reverse (or sometimes 'forward-proxy' if there's a need to disambiguate from a reverse-proxy) is somewhat specific to web proxies or other network protocol proxies. This is in contrast to say, a legal or rhetorical proxy.