Hi, Discord server is not fully closed, anyone can join (you just need to meet Discord TOS terms) . Punishments are not handed out "just like that", there must be a reason, I haven't noticed users being banned just for "fun".
Various updates, changes, and information are provided via the announcement channel https://forum.simrail.eu/forum/124-announcements/, which is shared from Discord here on the forum. I also see that developers respond to some bug reports here on the forum and interact with the community.
I'll add my PERSONAL opinion - In recent years, the "culture" on the internet has changed dramatically, and people from forums have migrated to platforms such as Discord, because communication is faster there and forums are generally "dying" and experiencing lower activity than for example i remember from 10 years ago . I have led/been part of active forums in the past, and today they are practically "dead" - which is not the case with this forum, where user activity is good, but communication via Discord is still faster, and that's just how it is.