Build couchbase-server-7.1.0-2209 contains kv_engine commit 422c4ae with commit message:
MB-25813: Fixup test case and remove @todo

So there's no behaviour change in the server in Neo - attempting to insert a non-primitive type via ArrayAddUnique returns status code SubdocValueCantinsert; just as it did in prior versions.

If the SDK is making changes to handle this scenario in a different way - it sounds like they are now checking for the argument type client-side and not sending it to the server if not primitive - then that is a behaviour change in the SDK and hence should be release-noted in the appropriate SDK release(s).

Thanks Dave Rigby that's helpful context, I may have misunderstood the ask from SDK team here, will refer back to them!