When Yo appeared, I first thought the app was pointless. You could only send somebody a 'Yo'. After thinking about it, I saw the appeal: sometimes a message needs no explanation if sender and recipient already know what the signal means.
Context makes an alert useful
In my original article I cited examples given by founder Or Arbel. A friend could signal that they want to meet. A colleague could announce a break. A shop could indicate that an order is ready. The word is nearly the same every time, but the agreement behind it gives it meaning. Without that agreement, a bare notification is confusing.
For a business there is another condition: people should choose which signals they receive. An update about an expected order may be useful. An alert for every random offer soon feels like advertising. Simplicity works only when recipients have control and the purpose is clear.
An idea from its time
The user counts, investment figures and app stores I mentioned years ago are historical, not reasons to install Yo now. I use the example to show a design principle. For every alert, ask: must this be a notification, what can the recipient do with it and is its meaning clear without explanation? Sometimes one agreed signal is enough. Often silence is better.