I think feature requests are better done via github: https://github.com/opnsense/core/issues
Has someone tried this new feature successfully with 19.1.9?
Not yet, waiting for 19.7
Why? It's officially part of 19.1.9 already. I've tested it successfully with Samba 4. It wasn't working with OpenLDAP because there you have to request (at least in my configuration) explicitly the memberOf attribute.