Would people find it useful to have an option that turns deprecation warnings into errors? I suspect it's easy to miss deprecation warnings when upgrading, e.g. in CI logs

Should this even be configurable? Some things are worthy of just doing, period, without adding a user setting.

Yes, it's important that we don't break your build from one release to the next. We use a warning and then only after that release can we change to erroring https://www.pantsbuild.org/docs/deprecation-policy

Fair