This app was mentioned in 4 comments, with an average of 1.00 upvote
Functionally, yes. I can't imagine any other use for it. It comes on when I plug in the charging cable and I was able to configure it to light up when I get a Calendar notification. I used this app to set it up/enable it after I rooted the phone.
No. None of the "rootless" apps (that I tried several days ago) saw the light. The first one I tried after This One worked fine.
It's a small light and the brightness can only be varied a little bit and it's just "white" (more yellow actually). I'm not really sure it's worth it by itself. I wanted some other programs (Titanium Backup, Root Explorer, Quick Reboot, etc.) too.
As to unlocking the bootloader voiding your warranty, it's true that they can cite that reason, but they probably wouldn't if the issue was unrelated to that. And I'm pretty sure the Device Protection coverage (which I have) doesn't care.
This app seems to work fine.
It allows you to configure which apps (and the light doesn't stay on all the time). I enabled Calendar and it notified me properly. When I "cleared" the notification on the status bar, the light went out.
https://play.google.com/store/apps/details?id=br.com.itsmeton.motoledenabler
Little bit buggy, but does the job for me :)