Currently feed subscription requires that the full feed url to be provided. However, websites have a mechanism to advertise the existence of a feed using ...
<link rel="alternate" type="application/atom+xml" title="Feed Name" href="/feed/blog.xml" />
... and typically may be enabled by default, even when there is no feed option displayed on the website itself.
It would be good to update the feed subscription panel where the input is the simply the website, and not the full feed url, and it parses the above metadata to find and subscribe to the actual feed url.
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https://www.rssboard.org/rss-autodiscovery for the spec. Active since 2006 or so.
P.S. Autodiscovery feels like a good step to a separate feedreader experience outside of the email inbox. A place for newsletters, blog updates, podcasts, etc. Detect whether inbox emails point to a web site with a feed and offer to (subscribe or automatically subscribe).