But in that case it should have a note saying it's finished or in maintenance mode (e.g. https://github.com/sirupsen/logrus); include references to replacements, offer paid support if you really need it or still use it, keep an eye on issues, and update dependencies.

Else, ask for a new maintainer. While code can be considered done (especially if no new features are added), it should never go unmaintained. If it's actually used a lot of course.