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Simple Update Protocol, or SUP, is a protocol developed by FriendFeed to simplify and speed up RSS and Atom. A number of websites have started to provide support for SUP, meaning that updates from their services appear in FriendFeed within seconds. These sites include Disqus, Identi.ca, reddit.
Brightkite supported SUP but was itself shutdown in April 2012 Brightkite.
YouTube supported SUP according to YouTube v2 API SUP documentation, yet as that reference says "The YouTube Data API (v2) has been officially deprecated as of March 4, 2014."
YouTube Data API (v3) supports push notifications via PubSubHubbub instead and no longer has SUP section.