Who develops & maintains the Roll-Up Reporting plugin?

The plugin is developed and maintained by InnoCraft, the company from the makers of Matomo (Piwik).
At InnoCraft, passionate product designers and engineers build and maintain the free and open source project Matomo. This ensures the highest quality and compatibility of all their plugins. As a result, popular Matomo features such as Segmentation, Scheduled reports and others will work out of the box.