@henryiii shared some useful data on common Black configurations in pyproject.toml files found on PyPI. A few pieces of weirdness stood out:

• Some people set line-length to a string, e.g. line-length = "100". This is very rare, about 25 instances total out of almost 20k (~0.1%).
• We allow both line_length and line-length. In Henry's sample, line_length was used about 3% as much as line-length. Our documentation only uses the hyphenated versions.

This might cause problems when we add a JSON schema (#4160), because it's harder to provide useful feedback to users if anything is allowed. For the typing issue, it also just seems confusing to allow setting an integer value to a string.

Proposed actions:

• In the documentation, state clearly that the hyphenated versions are preferred and that you should use the right type.
• Print a warning if we encounter the incorrect versions, and possibly remove support for them in Black 25 next year. I feel this is definitely appropriate for values of the wrong type. For the underscored versions, I feel deprecation is less justifiable as they're more common in practice and less likely to lead to confusion. Curious to hear other opinions on whether that is worth deprecating.