Behance

Behance's autodiscoverable RSS Feeds do not work*, u/gurkoz found that there are working hidden feeds:

"https://www.behance.net/feeds/user?username=" + behanceUserName

Example for user bernhardlang:
https://www.behance.net/feeds/user?username=bernhardlang

* All Behance profile pages show the same RSS Feed (http://feeds.feedburner.com/behance/vorr) which, although it looks like the feed for the user vorr, it contains posts from all users. I find it useful for discovering new artists.

DeviantArt

DeviantArt doesn't have RSS autodiscovery but they have RSS Feeds. From their documentation):

"https://backend.deviantart.com/rss.xml?type=deviation&q=by%3A" + deviantArtUsername + "+sort%3Atime+meta%3Aall"

Example for user loish:
https://backend.deviantart.com/rss.xml?type=deviation&q=by%3Aloish+sort%3Atime+meta%3Aall

Do you know of other sites with hidden RSS feeds?