Number (and age if possible) of open tickets might show if someone's paying attention to the dist. Like David said, much like the adoption criteria. The issues don't have to be valid, they could even be spam for all it matters, as long as someone's taking care of them.This is a tricky issue, as I found when trying to tune the adoption criteria. There are plenty of big name dists that have a lot of open issues, and always do.
My current thought on this is that if no issues are getting dealt with in some timeframe, then it fails the metric. Even if a dist has a pile of open issues, if at least some issues are getting dealt with, then as you show, that indicates some level of maintainer engagement. That still has failure modes though: someone might have adopted a dist that they’re really not up to maintaining, so they avoid the large / scary / critical issues.