The following entities,
are promoted to aggregate roots.
The only entity that has such child collections is the Parties entity.
What to do if this change affects me?
You just have to get the "collection" you need, referring its entity.
E.g. instead of this:
You should do this:
GET ~/Crm_Customers?$filter=Party eq 'General_Contacts_Parties(99344e29-5049-df11-a1e3-0018f3790817)'&$select=DEFAULT