What happened to Shared Links in Safari 11.0
You can find a plist of your feeds at ~/Library/Safari/WebFeedSources.plist. Use this command to get a list in a human-readable format:

plutil -p ~/Library/Safari/WebFeedSources.plist


It is annoying that the Shared Links feature was removed in Safari 11 -- Apple seems to add/remove RSS support every few years...