Reading the spec another time, I noticed this statement:

A “resource identifier object” MUST contain a type member. It MUST also contain an id member, except when it represents a new resource to be created on the server. In this case, a lid member MUST be included that identifies the new resource.

In which case could a resource identifier object be used to create a resource?

If I haven't missed anything, base specification only supports one way to create resources described in 9.1 Creating Resources. And that one requires resource objects.

A resource can be created by sending a POST request to a URL that represents a collection of resources. The request MUST include a single resource object as primary data. The resource object MUST contain at least a type member.

This was introduced in #1244. Adding it to v1.1 milestone as we may want to revisit before releasing stable version.