Custom Sleep/Lock Screen in a rooted device (no tutorial here, sorry).
Doing some tests I discovered the images for power off, sleep screen (or what users here call lock screen due to the lock icon) live in Kernel. Four of them: sleep screen with the padlock, logo, powered off, and flat battery. Plain 480×800 bitmaps, no files anywhere. (drivers/misc/mudita/eink/).
Problem is you can’t just rebuild the kernel with your own. Mudita’s published source is missing the eink/ folder that holds them. So the workaround is a small Magisk-loaded kernel module that swaps them in memory at runtime. Reboot restores the originals, so it re-applies each boot.
You can also tell the kernel to paint the power off image any time, and it doesn’t actually power off, it just draws. So that slot works as a repaint channel, letting notifications show up on the sleep screen without waking the phone. (kinda like an Always on display, which this technically it’s not).
In addition to the text notification it turns on the LED (green for whatsapp, blue for signal and red for anything else).
Battery cost for these features is negligible. Nothing runs on a timer, the screen only draws when a notification arrives, which already wakes the phone anyway. Once it’s drawn the phone goes straight back to sleep. The LED draws almost nothing and switches off as soon as you check the phone or you view the notification in aother device (phone, pc).
Please don’t ask for a tutorial, this was just to show that the sleep/locked display can do much more than just show you a lock icon.