Sometimes I send a review to a co-worker who is on vacation. It would be nice if phabricator could know this somehow, and tell me that all my reviewers are unavailable. Additionally, if I had multiple reviewers on a project, it would be nice if the differential UI would show that some of the reviewers were currently unavailable (maybe by putting a red dot next to their name).
I have no idea how this would work. My thinking is that phabricator would associate, with every username in the system, time-ranges where that user was unavailable for reviews. And there would be some way where you could add such a timerange via the UI and via the API.
In our company, we mark days we're on vacation on a calendar, so then we'd have to write a script that scraped the calendar and imported the data into phabricator, which is annoying (due to permissions) but doable. But I don't know if it's so annoying that this feature wouldn't be generally useful. I guess people could still enter that info in by hand.