It's in a mature state, already at version 8. Given that it's a minimalist framework this isn't as much of an issue as it would be for Bootstrap or something similar.

This question would have to be answered by https://twitter.com/jxnblk though

Edit: I asked him on Twitter

> I'd consider it feature-complete, but not unmaintained. Most of the open issues are minor and just need to be triaged https://twitter.com/jxnblk/status/860208848359088129