http://protonmailrmez3lotccipshtkleegetolb73fuirgj7r4o4vfu7ozyd.onion/blog/protonmail-open-source
While it would have also been possible to open source Proton Mail 1.x, we felt that such a move was not appropriate given that the code was intended to be deprecated. By open sourcing Proton Mail 2.0, we are open sourcing the future of Proton Mail. As we continue to expand our private email service with mobile apps, you can look forward to more open source announcements as our code base matures.