It took me a long time to figure out that the way to get my playlists into the playlists sidebar was by starring them. In my opinion this is not obvious as the playlist bar is not called "starred playlists".
However what I think would be even nicer would be to be able to either drag and drop a playlist file (or folder of playlist files) into the playlist sidebar to add it.
At first I mistakenly assumed that "playlist > add folder" was add a folder of playlists to the playlist sidebar. Maybe that option could be renamed "add folder to playlist" to make it clear what it actually does, and "add folder of playlists" could be a new menu option.
Also, just wanted to say that I love Strawberry and it has been my main music player for years. Thank you for improving on and continuing to maintain Clementine (as Strawberry)!