Migrated from 9.20 to 20.4CU5 yesterday and all works fine, except the dashboard view for NimBUS users.
To simulate: I created an IM NimBUS user with Superuser rights.
In the Operator Console, in the Dashboard Designer, the user can access/view ALL dashboards.
The user can access/see dashboards in the Dashboards view that are published to public/private visibility,
but the user is a Superuser and should be able to view/see ALL Dashboards yet this user can only view 6 dashboards.