mclsza I decided to try this app and managed to install it. As I expected from reading the information on the website, the app requires the Accessibility permission in order to do its intended function.
I deeply respect that some users have a need to use accessibility services. But in this case, I don't see any benefit, as the app doesn't seem to deliver any features that are useful in any way. From the app's website, I fail to grasp how it actually delivers the advertised encryption. I gather that it requires both sender and recipient to be using this app? In that case, why not use established and trusted end-to-end encrypted messaging services, such as Signal?
The Accessibility permission gives an app an extreme amount of control over your device. I recommend reading a further explanation of this here: https://discuss.grapheneos.org/d/11719-fr-toggle-flashlight-with-long-press-powerbutton-with-display-off/10
https://discuss.grapheneos.org/d/11719-fr-toggle-flashlight-with-long-press-powerbutton-with-display-off/22
I usually don't discourage people from using certain apps, unless they are clearly malicious. But in this case I recommend steering away. There's no reason to let this app control your device.