I've tried out some of the libraries, and it looks like they do calculate the difference between the installed version and the last (stable) release. If a dependency hasn't seen a release in ten years, those ten years don't count against the dependency drift. This is exactly what I would want.

However, they only check openly accessible (i.e. OSS) dependencies. If one of those hasn't seen a release in ten years, I would look for an alternative.