The old API is deprecated from 1.0.0, please use the new API.
We focused for the 1.0.0 release at making ViaVersion multiplatform compatible. This also meant we had to change the way ViaVersion and the api worked,
The class ViaVersion is deprecated, by usages like ViaVersion.getInstance() use Via.getAPI() intead.
ViaVersion provides a Maven repository that you can use to implement the ViaVersion API inside your project.