The Experimental Sidebar Feature set, including custom collapsible categories and unread channel filtering, is available in Mattermost v5.26+ (August 16th, 2020) under an opt-in config setting that can be enabled by System Admins:
https://docs.mattermost.com/administration/config-settings.html#experimental-channel-sidebar-experimentalIf you are currently testing the Experimental Sidebar Features, including collapsible categories and unread filtering, we'd appreciate your feedback in the brief survey below as we work hard on delivering an improved sidebar experience.