The answer does indeed apply to those things! For essentially the same reason, too (albeit not quite identical).
Just like file_server, the reverse_proxy does not alter the URI in any way in conjunction with matchers by themselves.
That means if you make your clients prefix requests for the RSS reader with /rss, when Caddy proxies it, it will proxy /rss upstream and your upstream server might get a bit confused if it’s not expecting that. Many upstream apps don’t!
Unfortunately, that’s not the only problem you’ll run into when reverse proxying, though. If your upstream server has links to static resources like stylesheets, images etc. you might run into what I’ve written about in the past as the “subfolder problem”. Have a gander: