why is there no low charge warning on magic mouse and keyboard now

If you have configured the Bluetooth preference panel to place its icon on the menu extras bar, then clicking on that will inform you of your Apple Bluetooth device charge status:

This should be the only heads up that one needs to monitor their device charge status. The Apple Bluetooth keyboard can charge while it is being used too, so if you don't want to be bothered by checking the above panel periodically, then just use the keyboard as a permanent wired device.

I also received notification for my Magic Mouse I. I only receive a message when the battery level drops below 10%. The Magic Keyboard charge lasts for a very long time so I haven't received as messages as yet.

- Pie Lover

I just got one, yesterday, for my Trackpad. I can’t seem to find any settings that would control that notification.

You are welcome.

I think any low battery warning now happens below 10% and because I monitor these devices, they get recharged before I reach the probable warning threshold.