How to deprecate an unmaintained repository?

What is the best practice to deprecate existing repository?
I don’t want to delete existing tag so we won’t break existing applications but I do want to encourage users to switch to another repository.

You can change the short description and add “DEPRECATED” at the beginning so everyone can see it. Add a link to the long description pointing to the new repository. Optionally you can release a last version which throws a warning to the log (standard error or standard output) like “This repository is deprecated. Use newuser/newrepo instead.”