One Tap Lock: Open Source

Lock is a tiny, focused utility that lets you lock your Android screen quickly from the app icon or home screen widget.

No extra menus. No ads. No tracking. Just tap to lock.

Source Code: https://github.com/RealQuickLtd/lock

Accessibility Permission:

This permission is required for the app in order to lock the screen, which is the only functionality of this app.

Here's a short video in action while using the permission:

https://drive.google.com/file/d/1yinGKZgNb3GWSO3QY4FSZwyCD_CQs_kJ/view?usp=sharing