I also suggested this to OutSystems, and the nice thing is the deployment plan can execute the deployment of the apps in the plan in parallel, as each app deployment runs in a separate container.

Yes, some small note will be helpful to understand the features/Purpose of deployment.

I verified that Deployment/Release Notes are already exposed through the ODC REST API

It seems that what we're missing is an additional input field in the Version popup during the Analyze Impact step of the deployment workflow, so that Release Notes can be provided directly from the UI.

Like @felipe.castilla said, it looks like we only need a field in the Portal UI since the backend work seems complete. 

Agree with @Daniël Kuhlmann that we absolutely need something similar to a deployment plan in O11 so we can easily group applications to deploy together. This would also mean that it's easy to reuse a plan or to rollback all apps to the previous version.