Make the DOM4J dependency optional in hibernate-core
Don't use DOM4J-based XML mapping at all *by default* (currently the default is DOM4J and JAXB is opt-in)
Exclude the DOM4J dependency from tests by default
Test opt-in DOM4J-based XML mapping through a separate CI job
Note we will have to keep the DOM4J dependency in envers for now. But we should be able to get rid of it in the core.
Actually, thinking about it... It's much easier to do directly in Hibernate ORM 5.5, and since it's a new minor we can afford that kind of risky implementation change.
Originally the plan was to introduce JAXB-based XML mapping in 5.4, where it would have been too risky to change the default. But in 5.5, I think it's a legitimate move.
So, closing this: I will work on in Hibernate ORM 5.5 instead.