I think we need to be noisy in 2.99 is the way to go. That way people are aware that they need to either lean on the formatter maintainers or remove it before going to 3.0. For 3.0, I think we need to `exit`.

I was on the fence about exiting, but that was only when I was thinking about formatters that are basic text alternatives to `progress` and `documentation`. However, for more complex CI or HTML alternatives, I'm not sure we can pick an appropriate fallback. I'd rather just stop, failing the build, then attempt to make a potentially wrong decision.