I think the rule of thumb is that any old product which doesn't firmly belong to one of the 5 units (Search/Knowledge, Chrome/Android/Google Apps, Identity/Google+, Advertising/Commerce and Youtube/Video) created after Page's 2011 company reorg is at danger of being shuttered. As long as a product belongs somewhere it has a better chance of staying alive, and one way to measure that is when the last update to it was.