Requirement:
We are deploying a fleet of microservices and utilize S2I Build, Image Streams and Deployment Configs with Image Change Triggers.
This way, every time a new Image becomes available, the Deployment Configs get updated with the latest Image Information pointing to an exact Image-Hash in Quay (this works with OCP4).
Sometimes (especially for production) we do not automatically apply the updated images so there are images references in running deployment configs that are not referenced by the tags anymore.
It is also common to rollback deployments and then use the exact image that was used in the last deployment.

Current Solution:
We are using Openshift's Image Pruning Functionality to prune images that are no longer referenced by OpenShift Objects.
https://docs.openshift.com/container-platform/3.11/admin_guide/pruning_resources.html#pruning-images

Question:
How can this behaviour be achieved with OCP4 and the Quay Integration? Quay should be able to discard those images that are no longer referenced by any OpenShift Objects (there are mulitple clusters served by a shared quay instance) but all Images that are references MUST be retained.

We have investigated Quay Image Expiration: 
This way the most recent images on a tag are kept, but older revisions expire and get deleted. This does not work for us, as the images referenced by the Deployment Configs will get discarded.
We have seen rollbacks of the deployment (and Pod movements to other Nodes) fail because the image cannot be pulled from quay.