This is a design change and expected behavior within vRealize Operations 8.6 and later. Being an advanced and useful feature, we did still keep it as a URL which users can access directly.
To workaround the issue, the object relationship feature can be accessed directly through the below URL:
https://vRopsIPorFQDN/ui/index.action#configure/object-relationships
Note: Replace vRopsIPorFQDN with the IP or FQDN of your vRealize Operations IP or FQDN