1. If sdist consumers will need to regenerate an sdist?

They would need to compile from the source, e.g. clone from Git and make an sdist (or install) from that.

2. How many sdist consumers will likely be impacted?

I don't know. Depends on how fast people migrate to Python 3.8.

3. If impacted, how difficult would the regeneration be for a scientist who is moderately comfortable with compilation?

See (1.)

4. Is there any guidance or information that can be given to sdist consumers?

"Raise the issue with the sdist maintainers" is probably their best bet.

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Realistically, I expect those people to stay on Python 3.7- until their dependencies are updated to (and also tested with!) 3.8.

I've seen anecdotal evidence of this hitting people who want to use real-world projects to benchmark Python 3.8+: https://mail.python.org/archives/list/python-dev@python.org/message/TF2JI7G5ZMCGUMM3AWNSCQDYVFNRPMQ4/

If we "stop discussing and just [Apply PR 14193]", people wen't be blocked from trying out 3.8, even if sdist maintainers don't care about 3.8 yet.