For all the 3.0 versions of the maven-plugins we require to not depend on maven-compat anymore. However, the Maven distribution still requires maven-compat. A simple proof: remove lib/maven-compat-3.x.y and execute mvn validate.
You'll get the following exception:
Reason: there's only one implementation: o.a.m.r.l.LegacyRepositorySystem, which is part of maven-compat.