The Clipboard Project is a clipboard manager that works entirely in your terminal. It has tons of swanky features including this new one in 0.9.1 that lets you securely ignore copying passwords and other things like that!
Link to the code: https://github.com/Slackadays/Clipboard
Interesting that there is a flatpak build as well. I knew it was possible to run cli programs with flatpak but this the first programm i see that explicitly lists that as an option.