I haven't seen any Gamblig Platform who used Telegram for Promotions, most I know is that they use Discord.
Do you have any Idea why they don't have any Telegram Communities? and why does it seem they don't use this platform for their promotions?
Discord is better and it's build for players and gamers and gambling sites has a lot of gamer members I like Discord features better than Telegram because you can create sub topics and sub categories, and besides telegram is very much associated to Cryptocurrency projects.
Telegram is mostly use for new project that want to have easy communication to its community using thier channel or groups its can also help them to advertise their project by having that channel . while only few gambling sites use telegram as terms of communication. most of them prefer discord since thats the right Channel or social platform for any game developer or games players that want to talk something about the game.