Previously Tumblr used to support Pubsubhubbub. They seemed to have stopped somewhere along the line, discovery of a hub on a feed URL is failing.
The protocol says there should be a Link header, but that's nowhere to be found.
Does anyone have a clue how to find a Pubsubhubbub hub for a Tumblr feed?
Apparently Tumblr dropped Pubsubhubbub support so that's why it's not working anymore.