A site proposal I have been following for most of it's first year is coming to the twelve month mark (still in definition phase), the proposal has gathered 54 questions and more than 100 followers.
In my experience, proposed sites just disappear one day. When I posted about rebooting it now before what is there is lost, Robert responded that they are "closed at least one month before they are removed". But unless you are visiting the proposal regularly you don't notice it is closed, you ask your 5 questions, cast some votes, and there is really not much to call you back to the proposal until the next phase is announced.
I suggest that when a proposal is closed everyone who is following/committed be notified about it. There are a couple of posts here on Area 51 (not finding them at the moment) about the positives of rebooting a proposal. The Stack Exchange staff might want to include some notification criteria, but if there are 40+ Questions and 60+ followers it seems there is some hope of rekindling the proposal, so everyone should be notified to see if any interest still remains.
Note A closed proposal also falls off of your "Followed" list so unless you have saved a link to it, you are unlikely to randomly visit during the 30 day closed window.