Are delta images planned for openBalena?

From the openBalena github page:

openBalena balenaCloud
Device updates using full images Device updates using delta images

Is this feature a way to motivate users to go to balenaCloud?

Deltas require significant infrastructure and resources and is indeed one of the proprietary technologies intended for balenaCloud only. That said, deltas are primarily a balenaEngine feature and in theory one could make and deploy them as-a-service for their openBalena installation.