Some discussion in T466 and T4087.

I started typing up a longer-form discussion of this issue but it was like 20 pages long and super rambley and not very useful.

The short version is that we're very unlikely to implement a feature like this. We built some nagging stuff at Facebook and it was annoying and not effective. My experience is that issues with review promptness are almost never accidental (e.g., the reviewer forgot, or really was too busy) and are almost always the reviewer avoiding work they don't want to do. Sometimes this is a reasonable choice, but a large part of this problem is social/cultural, at least in the teams I've seen at Facebook.

In Phabricator, you can already do this with: Click Name > Send Message, which will send a Conpherence message, or by simply commenting. (When a username appears as "@username" in text, you can hover over the username and "Send Message" directly.) Outside of Phabricator, you can use email, chat/irc, physically poking people, etc. Adding another channel doesn't seem dramatically more useful than these existing channels.

Users can write mail rules to highlight messages which include their "@username" if they're having trouble managing mail volume. I'm generally not very sympathetic to this complaint, as I'm not sure it's a real phenomenon. When I worked at Facebook and had access to the mail logs on that install, I had more than 10x the volume of the engineers who were the most vocal complainers about mail volume.

The one feature here which I think may be worth doing is putting some kind of special highlight/notification on @mention notifications, in the Notification list. This would mostly cover the "just got back from vacation and have 200 things to look through" case, since it would let you identify the personal ones more quickly. But you can already do this with a simple mail rule in your mail client.