>I don't think there's any at least medium sized project that is considered to be done but not abandoned/legacy and does not receive any commits.

As another commenter mentioned, this can be pretty common in math libs.

Here's an example: https://github.com/fommil/netlib-java

This project BLAS/LAPACK/ARPACK bindings for JVM languages. The repository is marked as archived and the owner explicitly states that the project is done. The last commit was 21 Jun 2017.

This project is still heavily used, there are still multiple libraries that use those bindings.