I am in the rather lengthy process of open sourcing a library for orbit propagation, but the previous art which is somewhat modern is the python rebound package. It's original design intent was for asteroid collision simulations, but it has been generalized a lot since then.

Most of the state of the art in the field is algorithms from the 60s/70s, the classic software which many people use are packages like Mercury, written in either fortran or C++.

These factors are why I am attempting to release my code to the (small) community.