Absent a $select, services are allowed to return a service-specific default set of properties.

Can that service-specific default set of properties change over time?

Adding new properties should be allowed, but what about removing properties from the default list. Unless we explicitly call out that services can remove properties from the default set, we could break apps that were expecting the same set of properties on each request.

However, if an app is expecting certain properties then they should be using $select to ensure they get those properties.