There have been many questions and uncertainty around the future of PyMC3 since Theano stopped getting developed by the original authors, and we started experiments with PyMC4.
We are happy to announce that PyMC3 on Theano (which we are developing further here: https://github.com/pymc-devs/Theano-PyMC)
with a new JAX backend is the future. PyMC4 will not be developed further.
See the here for the full announcement: https://pymc-devs.medium.com/the-future-of-pymc3-or-theano-is-dead-long-live-theano-d8005f8a0e9b