Since the tag parsing feature is opt-in it may be worth it to make at an opt-in feature of the lint command too.

I did initially add a new option to the command to enable the flag, but wasn't sure if that would count as a new feature then instead of a bug fix?

Seems like a feature yes, although it should have been added at time the constant has been introduced.

Linting the yaml files is part of our build process, so all our builds keeps failing because of this. So if this can only go in as a new feature in 4.1, then we won't be able to lint yaml files until May next year and after upgrading to Symfony 4 (or we must duplicate the logic in our application) unless this can be enabled by default without the additional option, but that might cause some false-positives if someone hasn't enabled custom tags

Thank you@pierredup.