Hi Kern,
The delete orphans is not deprecated. Indeed in the documentation is mentioned only under the old section but that is because by default this functionality is not yet supported by the designer. We have plans to introduce support for the delete orphans functionality in a future release.
As for now the only way to set is via the fluent api or in the metadata container directly where the option is present.
All the best,
Petar
the Telerik team