NationStates

How often is NationStates updated? Like actual content added to the game?
And I don’t mean issues

It varies, depending on how busy the Admins (who are unpaid volunteers, with lives to live outside of NS, too) are, how easy to code whatever ideas they decide to add are, and indeed whether any new ideas that are both worth adding and practical to code occur to them or are brought to their attention. The only forecast that can be made with reasonable confidence, on the basis of many years' precedent, is that there will [probably] be either a new 'April Fools' or a new minigame on the 1st of April.
If you skim back through the 'News' thread you'll see what was added when.

It looks like the last time new stats were added was fairly recently
That’s good for me, I guess