Without any big announcements, Microsoft started rolling out support for cross-premises Send on behalf of and folder level permissions. Here's the Roadmap item:
And here are the updated TechNet articles:
Rollout is expected to finish by end of April. Send As permissions are still not supported, neither is Automapping. And here's also one KB article with troubleshooting information: https://support.microsoft.com/gl-es/help/3064053/overview-of-delegation-in-an-office-365-hybrid-envi...