The left sidebar contains options to open the Subscriptions, Playlists and History pages. However clicking these don't actually do anything. This happens using PlasmaTube compiled from git master yesterday.
For me, the Subscriptions are simply grayed out, even for a "piped" video source where videos show up. When I click it or hover it, there is no explanation why. I don't know if it's the same issue, but perhaps the UI might want to explain when it's unavailable for a good reason if it's not simply a bug. Perhaps the Subscriptions page is just generally buggy though, and this is the same problem.