> It's probably worth at least a post to python-dev to expand the audience, but unless someone speaks up with some really good reason why they must update to Python 3.10 without updating their own code then let's deprecate it.

I don't think that it deserves to be discussed on python-dev. I propose PR 18396 to add a deprecation in Python 3.9. We can open a discussion once we will reach the point of actually removing the feature. It's easy to revert a deprecation if needed.