IANAL, but AFAIK that’s incorrect. If you’re the only copyright holder, you can issue multiple licenses for your work. GPL doesn’t allow you to rescind previous issues, so anyone in possession of your GPL code can still modify and release it under the GPL freely. But it doesn’t prevent you from issuing your own work under a different license.
There isn’t usually much economic sense for most applications to do that because anyone can fork the project and distribute it for free. For Proton, since they still hold the server as closed source, they could simply introduce a breaking protocol change and all the forks would be useless.
IANAL, but AFAIK that’s incorrect. If you’re the only copyright holder, you can issue multiple licenses for your work. GPL doesn’t allow you to rescind previous issues, so anyone in possession of your GPL code can still modify and release it under the GPL freely. But it doesn’t prevent you from issuing your own work under a different license.
There isn’t usually much economic sense for most applications to do that because anyone can fork the project and distribute it for free. For Proton, since they still hold the server as closed source, they could simply introduce a breaking protocol change and all the forks would be useless.