The forum is better for information that should exist for more than 5 seconds. Things like proposals, thoughts, etc are better here. People will see your thread when they have time to visit the forum, hours, days or even years after you post it.
Telegram & discord are real time chat. The only people that will see your messages are people that happen to be online at the time. Very few people re-read 10s of thousands of unread messages on discord & telegram. I certainly don’t!