I suggest Hibernate ORM 5.6 (used in Quarkus), 6.0 (used in WildFly) and possibly also the main branch adopts a tiny change that allows compiling with Jandex 2.4 and running with both Jandex 2.4 and 3.0.

The only change in Jandex 3.0 relevant to Hibernate ORM is that Indexer.index() used to return ClassInfo, but now returns void. This is a breaking change, but Jandex 3.0 has a synthetic bridge method with the old signature for binary compatibility – except it always return null. Therefore, Hibernate ORM either needs to ignore the return value completely (preferrable, as that makes it source-compatible with Jandex 3.0), or be prepared to handle null.

This is not a duplicate of https://hibernate.atlassian.net/browse/HHH-15465. That issue suggests direct upgrade, which is a breaking change. In my opinion, that requires at least a minor release and isn’t suitable for backporting. In that sense, this issue is a short-term thing that can be included in a micro release, while the other is more of a long-term kind of change that requires at least a minor release in my opinion.