It's effectively a pause. In a project the size of CPython, and a subproject the complexity of a JIT, you can't continue work on a separate branch/repo without guaranteeing that there will be a massive amount of (both textual and semantic) merge conflicts down the road.
But with a project running for as long as they are, is it not already clear what they are going for? Is the architecture not settled?
If it is, is it not documented somewhere? Maybe as a formal PEP?
If it is not and it is still in heavy experimentation phase (which is fine), it should not be part of the mainline CPython no matter how much more effort it is for the team to experiment with.
That works, until that inevitable one merge that's harder to fix and takes longer, which in my experience then tends to snowball until it's basically the very merge hell you were trying to avoid. Can't say I've ever had a great experience with long lived feature branches. I can't imagine what it would even look like trying to do this on such a massive project and such an overarching feature.
