Android OS contains a default Keyboard and KeyboardView implementations (deprecated as of Android 10, but still available). Input method developers can use these classes as base for their own keyboard implementations. Tiny Keyboard is an implementation without any changes.

Smallest possible APK size of 22kB (as of version 0.3)
Permissions: 0
Features: it has keys
Supported layouts: en_US
No Launcher icon and Settings
To enable the keyboard:

Go to: System Settings - Language and Input - Virtual