My mistake. But not mine only.

According to https://www.python.org/dev/peps/pep-0572/ the PEP is Python-Version: 3.8

I took this to mean that it had been implemented in Python 3.8. Reading the PEP more closely, it also says Status: Accepted.

According to https://www.python.org/dev/peps/pep-0001/#pep-header-preamble each PEP can have an optional Python-Version, which is "described below". I thus find
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Standards Track PEPs will typically have a Python-Version header which indicates the version of Python that the feature will be released with.
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I seems to me that there is a bug in the PEP, perhaps caused either by a failure to increment its Python-Version as part of the release of Python 3.8, or the PEP itself providing too early the optional Python-Version.

I'll email Chris Angelico about this. I'm very happy for the issue to remain closed.