The following improvements are included in this release of the Web API, our OData v4 endpoint:
Custom actions that return EntityReference, Entity, or EntityCollection types are available.
Changes to API behaviors are available using the latest v9.0 version of the service, legacy behaviors remain available in the v8.x version. You don’t have to change your code when you upgrade.
New messages: GrantAccess, ModifyAccess, and RetrieveSharedPrincipalsAndAccess messages are now available using the Web API.
We have made the amount of service metadata smaller by not including annotations by default. If you need the annotations, you can use parameters to have it included.
Hope it helps.
Source: Microsoft
