I had a customer notice that the Addon Domains icon no longer appeared in the control panel. So I checked it and he was correct even though nothing had changed on our end, and it was enabled in the Feature Manager for the package that was assigned to his account.
In checking other themes I found that the Addon Domains icon or link no longer appears in them either. So on a hunch I did the following which makes it work again....
In the /usr/local/cpanel/base/frontend/x/index.html file there is line that looks like this:
I changed it to the following below: (removed the "s")
Now it works again since I made the change. I'm not sure at what build this changed. I am running WHM 9.1.0 cPanel9.1.0-S93. I just thought I would mention it.