Seems like Black updated what they consider to be "preview". I got it to run using the options --preview --enable-unstable-feature string_processing:

black -l 80 -t py38 --preview --enable-unstable-feature string_processing -c 'long = "This is a long line that is longer than 88 characters. I expect Black to shorten this line length."'

long = (
    "This is a long line that is longer than 88 characters. I expect Black to"
    " shorten this line length."
)

NOTE: The last comment/answer to https://github.com/psf/black/issues/1802 (referenced in @eka's answer) doesn't seem work with Black version 24.8.0.