Probably only a few, but there are web servers out there where it would be a minimal effort to implement it, which are sadly only a few too.
It's always confusing me how a Feed Reader looks like a normal desktop browser when it loads a feed, e.g. Accept: text/html instead of application/rss+xml first.
Most feed readers don't even tell the server which format they handle at all, most just send a Accept: text/html, /
I've tried Github release page, but it doesn't work.
For RSS Atom "application/rss+xml" is non-standard