In my view, it is a functionality that was there in previous and other clients and is missing on the relaunch of the web client.
To be honest, I don't know how I added them. It seems that there isn't a way to add them to the Shortcut list in the iOS client.
Now, I think, perhaps it's a deprecated functionality. 😞
I use it very often. Evernote is my GTD and TODO list. Having tags like "Next" and "Due" on my shortcut list is one of my most frequents usages of evernote. 😕