They were replaced by "always on" OLED displays. When I turn my phone off, the screen still displays the time and notifications. The beauty of OLED is that each pixel is its own LED, so only a portion of the screen needs to be powered. Essentially, the whole display is the new notification light.
Late nineties ,early aughts.. Did you used to mod Nokia bricks and early Motorola flip phones? (Razr iirc) Used to get blinking LEDs, replace the antennas with them, clear batteries that had flashing lights. Pretty much the OG RGB