Michael Luckey

As org.gradle.api.internal.CollectionCallbackActionDecorator was added with gradle 5.1, this effectively means, that this latest version is only usable with gradle 5.1+. Is this intended or an oversight?

Yes, of course. But on the other hand, their might be constraints which prevent upgrading your project to gradle 5.1. It might be intended, but as the docs still state...