As the original author of the
discourse_telegram_auth I must say that for the most typical usecases my approach is really terrible; there’s definitely a market for an auth plugin for Discourse which allows “login with Telegram” just as one of the login options, side-by-side with Google, Github etc.
Doing a separate SSO add certain exploitation complexity (the need to run a separate server which handles the auth proxying — including having a public hostname and https setup for it), but also is not the most straightforward thing UX-wise.
I didn’t do any searching, though; maybe such plugins already exist. My code is not that that, though.