The current UX, at least on my iPhone, is that I go to a site that looks like it _probably_ has a feed, and then I use the share sheet to push that URL to NetNewsWire. Then RSS auto-discovery takes over.
But there are a growing number of sites that look like they _should_ have feeds (posts on the front page arranged chronologically) but don’t. And the user experience is simply terrible when you attempt to subscribe with auto-discovery and it silently fails.
The problem is getting that in-browser glowing button on mobile. Browsers are too locked down.