With introducing of Wave 1 2020 we are able to customize the Opportunity Closing Dialog, handled in entity "opportunityclose". Currently it is not possible to create a 1:N relationship (Lookup) from this entity to another one, since CanBeRelatedEntityInRelationship is disabled in entity metadata by the system.

From my POV this is a left-over from the restrictive mode on this entity. Can we also enable the functionality to create lookups on this entity?