When is/was soft typing of the Part Usage Link really available.

When is soft typing of the Part Usage Link really available. As of M030, it wasn't available.

All builds of 9.1 say that you can soft type the Part Usage Link, however, the interface doesn't allow you to create a "relationship constraint" to actually use the soft type. Has anyone created a custom Part Usage Link and what build code of 9.1 finally allowed it.