Examples:
The signature for setAllowedValues is different in OptionsResolver as well and there are probably a few other examples.
While OptionsResolverInterface is deprecated things like the Form component still use it. It would be great to know those new methods are available so changes could be made in in components that still depend on the interface now in preparation for its removal in 3.0. Adding methods to interfaces doesn't seem like a BC break, or, if it is, Symfony is already doing it.