For me it’s the notification light you used to find on older phones, was particularly good to know if your phone was charged without picking it up
I used to have a custom ROM that would allow me to change the color based on which app had the most recent notification
Even more than that, in early versions of Android this setting was baked in. I had colors set based on text messages, emails, etc. I think around 2.x was when the option was removed.
I remember and miss that. Any remember the original droid with the slide out keyboard ?