Why is it useful, who would benefit from it, how should it work?
A master release contains phases and milestones, along with the sub-releases which are part of it.
The phases and milestones of the master release are meant to be used to represent and help manage the combined delivery steps across all sub-releases.
Based on the above description, along with current priorities and community feedback, we are unlikely to make updates in this area. We hope you can understand.