How can I get my RSS feed to be Auto Discovered?
Attach the Feed display to a valid Page display. The "Master" display may not be a valid page display.

An example of this can be seen in the default "Front Page" view. In this view, the feed display is attached to page_1, which is the page at the URL /node.

See the YAML export snippet from views.view.frontpage.yml below:

  feed_1:
    display_plugin: feed
    id: feed_1
    display_title: Feed
    position: 2
    display_options:
      sitename_title: true
      path: rss.xml
      displays:
        page_1: page_1
        default: ''