As Andrea has noted on discussion of GEOS-8884:
Mind, even if the data dir reading gets addressed, the change will still break REST clients. The above attribute structure is only present sometimes in the serialized data dir, but it's always present in the REST output for feature type resources, e.g.:
So REST client relying on this information will be broken on upgrade. I don't think we had a
"migration guide" for GeoServer in years, but this time it will be unavoidable. Bummer...