There is no low battery alert

No. We're expected to manually check every so often and plug them in overnight. Low battery alerts are only on laptops and portables.

Tell Apple what missing features you'd like restored or new features added to the system via Feedback - macOS - Apple.  

You might look at an application called "Cloud Battery". It will monitor the battery level of devices connected by BT.

It takes a few seconds to check the battery status on both the keyboard and mouse. If the Bluetooth icon is showing on your menu in the upper right of the screen click it and the percentage of battery left will show. If you don't have that icon showing it is simple to add it. Open System Preferences - Bluetooth and look for the checkbox that asks if you want to display it.

I check mine about 1x every 2 weeks, note that everyone's batteries will run down at different rates based on the amount of use it gets. So if you are in front of your computer 8 hours a day and are constantly using keyboard and mouse your battery will run down faster than someone that is front of their computer 1/2 that time.

.

If you periodically click on the Bluetooth icon on your menu extras panel, or in the Control Center, you will see the remaining battery percentage for your Apple Magic Mouse 2 and Magic Keyboard. The latter can recharge as you continue to use it while it is connected with the recharging cable that came with it to a power source (e.g. hub, etc.).