"No longer maintained" is hard to define, does no commits in the last year mean "this project is mature and needs no changes", "this project is dead and should not be used" or "the author still cares about this project but is busy with other stuff this year" or something else?

Exactly. It is subjective. One just needs to look at the contribution history and determine if it is what they need.

Depends on what the repo is/problem it solves. Sometimes something is considered "done", when it doesn't need anymore work to it. :-)

How do you define if a small library has been stable for years?

Issues filed. Their recency and concerns.