> CI would fail on existing code that previously passed.
You problem isn’t that black changed formatting rules, your problem is that you didn’t pin CI dependencies. This isn’t a unique problem to black, any other tool used in CI pipelines can cause things to break if you don’t pin them.
It would be a stretch to say they demonstrated that it’s better to enforce a standard.
Black, itself, changed rules many times within a 1-year period to the point our own CI would fail on existing code that previously passed.