Hello, has Cesium for Unity provided a method for converting the J2000 coordinate system to a longitude, latitude, and altitude coordinate system?
I don’t think so. PROJ is a widely used open source library for coordinate reference system projections/conversions GitHub - OSGeo/PROJ: PROJ - Cartographic Projections and Coordinate Transformations Library. A quick google search yields that someone made open source bindings for C# GitHub - AmpScm/SharpProj: .Net / C# wrapping of PROJ. I would start there for your use.
No, there is currently no J2000 support in Cesium for Unity. You can likely find a library that converts J2000 to ECEF (a transformation that changes over time, because the Earth rotates), and then use Cesium for Unity to convert the ECEF coordinates to longitude-latitude-height or Unity coordinates. For example, I know Ansys DME Component Libraries can do this (I used to work on that product), and it can be easily called from C#. There are likely other options available.