Modern browsers happily show you the actual characters, while sending their encoded entities to the server. So, from a user perspective there is no ASCII limitation. Case in point: söhne.at (just some random website, I have no idea what they are or if they are legitimate)
Threema is very security focused at the expense of user experience. You lost your private key? Tough luck, here’s a new account and everyone will have to verify your new account again. I have had conversations with friends over five or six of their accounts because they use threema just like WhatsApp. I stopped recommending threema to folks without a basic technical understanding. Signal is more than good enough for their needs.
Modern browsers happily show you the actual characters, while sending their encoded entities to the server. So, from a user perspective there is no ASCII limitation. Case in point: söhne.at (just some random website, I have no idea what they are or if they are legitimate)