But are you sure? Assuming that this constraint is the only significant thing, I wouldn't mind giving people the option and simply refuse any mapping which would not respect this - if using 6.0.

I have a branch for that, last updated about a month ago.
Last build: https://travis-ci.org/yrodiere/hibernate-search/jobs/272017509
Results: Tests run: 1565, Failures: 53, Errors: 182, Skipped: 28

So, even if it's only about updating our tests (disabling some on ES6), there is still quite a bit of work left.
5.10 or 6.0, but definitely not 5.9 material in my opinion.