The source for the clients is available.
1. https://github.com/DrKLO/Telegram
2. https://github.com/TelegramMessenger/Telegram-iOS
All Telegram clients are already fully open. (The server side is not, but there are third party implementations).
https://github.com/telegramdesktop/tdesktop
https://github.com/DrKLO/Telegram
https://github.com/TelegramMessenger/Telegram-iOS
Plus a few web frontends, I am way too lazy to find them all.
https://github.com/zhukov/webogram
WhatsApp forbids third party clients IIRC.
Disabling issues is possible.[1] Telegram's official Android client repository disabled issues a few years ago.[2]
[1] https://docs.github.com/en/github/managing-your-work-on-gith...