While it seems you can now group features into master features and highlight features by master feature in the Releases gantt chart, you still cannot show just the master features under each collapse-able release. In order to show a higher-level view like this you have to create a custom roadmap report.
For ease of use (particularly for new users) it would be nice to have this view available as a default option in the Releases gantt chart.
Create one comprehensive view of your product plans. See epics nested in their releases on a brand-new Gantt chart.
https://www.aha.io/blog/new-gantt-chart-see-releases-epics-and-features-in-one-view