I remember my little Motorola flip phones from 25+ years ago had an LED that would blink for things like voice mails or unread SMS. I can't remember if it also did missed calls, and maybe could do orange or green? I seem to recall it did a unary pattern to blink out the number of queued events.

The problem with this kind of feature is that it falls apart if you're the type who leaves things unread or gets them all the time. Then, it is little more than a beacon to say "phone still exists"...