Hi all. Erstwhile ez-templates plugin maintainer here.
This is very poor situation indeed. InheritanceStep has never been a class of this plugin - the history is there for you to review and I share your frustration. At some point prior to ez-templates migrating into official jenkins repo, someone uploaded an unofficial hpi from their private fork. Presumably, this is the code you've been using.
I rejected that change as complicating ez internals too much considering I'm (ok very slowly) intending a refactoring that will make it much more powerful.
Please re-evaluate and drop support if required (but do create a new jira on both Components to add it back!)
update: confirmed http://repo.jenkins-ci.org/public/com/joelj/jenkins/ez-templates/ needs to be expunged - that's not code that made it past a PR. As per standard - when onboarded it became ez-templates_-plugin_ and I never noticed this other stuff camping on the previous name. I'll leave it a month or two before asking for it to be removed.