Content negotiation is a thing that should be used. My browser sends this in the request headers when I open up one of those RSS links:

    Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,*/*;q=0.8

Which means it would prefer to get a response from the server in (basically) the order shown, but it will accept any response type (*/*). So ideally the server would be using that information and making the decision to serve the RSS feed as application/xml instead of application/rss+xml. AFAIK if the "subtype" (rss+xml in this case) has a + in it, then it basically means it conforms to the format after the + (e.g. application/xml) and thus is just providing a bit more context, but is still valid as the more generic version.

Though I do think the browser should also keep that same thing in mind and attempt to render any application/whatever+xml in its XML renderer.

Edit: another thing you could try to add this response header:

    Content-Disposition: inline