One of our (or at least mine – @Nikerabbit) original ethe is that we must go to great lengths to avoid wasting translators' time. One of the worse kind of wasting is doing translations that will never reach the users. For this reason we should regularly check that all our supported projects are still active enough that new translations are incorporated and reach the users.
I suggest a yearly process to minimize the overhead.
For some kind of a loose criteria what is considered active:
* Has made a release (or deployment in case of websites) in the past year
* Has developments in the past year and has made a release in the past two years
Anything with no development or release activity whatsoever in two years should be removed right away and inform maintainers this has happened. If they take actions to rectify the situation, the project can be restored.
Anything with no release (or deployment) in the past year should be contacted and let them know that if they don't make translations available to users soon, they will be removed from translatewiki.net
Common sense is to be used to make decision case-by-case basis, these are not intended to be hard rules. While checking projects, it would be super helpful to make sure basic info on their project pages is up to date (repository location, license, contacts).