anonsubgift, anonsubmysterygift, anongiftpaidupgrade are documented but never occur
https://dev.twitch.tv/docs/irc/tags#usernotice-twitch-tags documents anonsubgift and anongiftpaidupgrade. I found this announcement post https://discuss.dev.twitch.tv/t/anonymous-sub-gifting-to-launch-11-15-launch-details/18683 saying that apparently it was temporarily disabled as a workaround. As far as I can tell to this day the workaround is used. The documentation should be updated to remove the originally planned but never put into service types, and instead document that anonymous gifts come from AnAnonymousGifter.