I created my first community to explore that feature. After that hide this community in the side bar. Now I can't find any setting to undo this hiding.
For the web app:
Go to Communities, and drag the desired one back to the Communities Panel as described in the text on that page.

Cool, there it is again. Thanks a lot!
But the interaction design of this feature should be questioned. The hide and re-add action are totally different. I think it would be easier to understand if the sidebar would have an additional button like 'Show all your communities here' in case you have any hidden community filter.
Yes, the entire UX of communities is planned to be redesigned. It's definitely confusing as it is today.
Bump on this, as I just ran into this issue myself.
Most helpful comment
Cool, there it is again. Thanks a lot!
But the interaction design of this feature should be questioned. The hide and re-add action are totally different. I think it would be easier to understand if the sidebar would have an additional button like 'Show all your communities here' in case you have any hidden community filter.