Hibernate ORM deprecated the old Criteria API in favour of the JPA Criteria API.
Hibernate Search uses the old one internally; not least this might generate warnings at runtime which can't be avoided by the end user.
It turns out it's not possible to migrate to use the JPA Criteria API without making breaking changes.
The main issue is that we expose the method org.hibernate.search.FullTextQuery.setCriteriaQuery(Criteria): if the user sets a custom Criteria, we'd needto convert that to a JPA Criteria to combine it with our additional restrictions. This conversion is not possible, or extremely tricky.
When Hibernate ORM will introduce SQM we'll have a better and converged representation, at that point we can re-evaluate getting rid of using the legacy Criteria internally.
For now we should just bypass logging the WARN, as end users can't do much about it: rephrased the issue title.