Missing Feature: Notifications for low-battery on Magic Keyboard/Trackpad/Mouse
I contacted Apple Support and got a confirmation that this isn't a bug. It's literally a missing feature, deliberately removed. Yes, you can monitor the battery levels on input devices via the Bluetooth menu. But it's easy enough to just forget to check, and then when you try to for example wake a sleeping Mac only to have nothing happen because the input device you're using has a flatlined battery.
I'm going to try to file a feedback report, but I can only imagine plenty of others are irked at this feature-removal. The more people who complain, the more likely Apple is to restore this feature.
Mac Studio, macOS 13.2