All Telegram client apps are open source, and licensed under GPLv3. Only Telegram’s server-side source code is proprietary.
End-to-end encryption is not necessarily a net positive: as discussed in the link above, it carries some costs over client-server/server-client encryption. Arguably, it’s better to give the user the option to decide when to use which, based on their weighing of the respective pros and cons, than to always enforce one option over the other.
A couple of clarifications:
All Telegram client apps are open source, and licensed under GPLv3. Only Telegram’s server-side source code is proprietary.
End-to-end encryption is not necessarily a net positive: as discussed in the link above, it carries some costs over client-server/server-client encryption. Arguably, it’s better to give the user the option to decide when to use which, based on their weighing of the respective pros and cons, than to always enforce one option over the other.