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Good points, but I think it's good to mention that Telegram is actually open source: https://telegram.org/apps#source-code

No, it's not. Telegram clients are; telegram, as a service - the server component - is not.

Further, the Android app doesn't have an issue tracker. According to [1] and links in there it was removed without explanation.

It's actually dated-source. The source code they occasionally publish is always months beyond the recent release.