Signal
Also on WindowsSignal's desktop app for private messaging, linked to your phone.
Installs from Signal's official download.
Good news: Signal runs on Linux. It's the same app you use on Windows, and TuxPick installs it for you from Signal's official download.
Install Signal with TuxPickIt works on Ubuntu 22.04 and 24.04, and on systems built on them like Zorin OS and Pop!_OS.