As far as I know, PyPy doesn't support all CPython extensions, so pure Python code will probably (very likely) run fine but for other things most bets are off. I believe PyPy also only supports up to 3.11?

Why shouldn't the reference implementation get JIT? Just because some other implementations already have it is no reason not to. That'd be like skipping list comprehensions because they already exist in CPython.

Because the same people who made a big deal about supporting PyPy and PEP 399 when it was fashionable to do so are now told by their corporations that PyPy does not matter. CPython only moves with what is currently fashionable, employer mandated and profitable.

PyPy is limited to maintenance mode due to a lack of funding/contributors. In the past, I think a few contributors or funding is what helped push "minor" PyPy versions. It's too bad PyPy couldn't take the federal funding the PSF threw away.

Because PyPy seems to be defunct. It hasn't updated for quite a while.

See https://github.com/numpy/numpy/issues/30416 for example. It's not being updated for compatibility with new versions of Python.