Where does this sentiment come from? I've seen it posted before.
I've been using Phabricator for 2 years and development seems to have picked up, if anything. They seem to be polishing what they have and making everything more robust, an odd thing to do if its going the way of the dodo.
Ah, I think that may be due to the patch submission structure of Phabricator more than anything else. I can understand why a major open source org wants to adopt something else that people have as muscle memory (ie. pull request style collaborating). I don't think that reflects the development of Phabricator at all, like I said, its become more robust and features are becoming more mature every week (and there's still some big installations out there, like Wikimedia).