Is telegram fully Open Source nowadays or is there some trickery involved?
Does The Reasons Why You Should Not Use Telegram Open Source Code (still?) apply?
That’s indeed a crux in context of Tor / Whonix.
Telegram is where many people who have been censored by private companies go nowadays and use it as a “hard”-to-censor [1] notification mechanism to reach their audience or modern replacement (IRC alike) chat channel. It looks very popular outside the Tor / Whonix sphere.
I would use Telegram to get the word out if I was censored [i.e. beyond already existing self-censorship] by private companies too. (At least for communications on subjects other than Tor / Whonix.) Even if there were “Open Source trickery issues”, which are really bad indeed, it does its job currently to provide free speech.
Related:
[1] I cannot personally say if telegram censors nothing, light, or whatnot since I did not research that in depth. From what I causally observed it looks like one of the most free speech supporting platforms at the momment.