> Why do we use a system property for this, instead of relying on the test base/setup classes?

As explained on github comments, that's not a reliable way to set environment properties: it might be too late depending on class definitions. Also it's not such a big thing that ORM would add additional boot options, not worth it.

> So is our intend to always use the latest services and avoid shadowing when possible?

It is our intent to not use code which is already deprecated at release date. The idea is that we need to be notified of such usages so that we can decide on a case by case without letting it "slip by" unnoticed, for example we could run the WildFly integration test without using the option in case WildFly where to use an older ORM version.