Users can use a browser plugin like Stylish to do this themselves if they desire. Of course, I'm sure it's fairly trivial to make a plugin that would do this.
How do you hide invisible users?
So on my forum, if you check which users are online, you can see people who have set their status to "invisible". When I try this on community.nodebb.org, all the invisible people disappear!
Does this witchery come with magical CSS? And if so, what are you doing?
Admins see users even if they set their status to offline.