When the RDA Toolkit beta site debut in June 2018, what had been relationship designators in the original toolkit became relationship elements. The Relationship Matrix was included in the beta site as a replacement for the appendices I-L that allows users to do a quick look-up of relationship elements. Unfortunately from the debut of the Relationship Matrix the feature has suffered from inaccuracies in its content and difficulties processing updates to the content.
It has become clear that the Relationship Matrix concept is simply not working. The Copyright Holders of RDA, in consultation with the RDA Steering Committee, have decided to remove the Relationship Matrix from the RDA Toolkit beta site, effective immediately. If the feature can be rebuilt and operate reliably, it will be returned to RDA Toolkit.
The recent revisions to the Elements list found on entity pages provides an excellent alternative to the Relationship Matrix. You can learn more about this feature in the April release announcement,