I know two of the commonly used free options for 3D vtubing are vnyan/vseeface:
https://github.com/Suvidriel/VNyanDoc (not sure what licensing it uses, but still uses unity)
https://github.com/emilianavt/VSeeFaceReleases (says it is not open-source)
Regardless, building custom models is likely still a lot of work and those wanting to get started quickly would probably still largely just buy pre-made models or hire someone to make a custom model. But it doesn't help people who want to get into model making specifically.
Live2D is like $100/year for 2D models. Not sure what else people use outside pngtubers.