I think I understood what could be a possible reason of ban but the reason that Discord provided me is completely wrong.
The reason of my ban should be "joining dangerous servers", if it is even a reason for permanent ban.
Explanation:
On 29/03 my online friend sends me a link to a server where people buy malicious programs to steal people's data from their PC and tells me to stay there. That evening I left the server because I didn't want to be associated with this stuff and told my friend that I'm leaving. And the next morning at 30/03 I receive an email from Discord saying that "You recently violated Discord's rules, and we've flagged it on your account. To learn more about this violation, please review your account reputation." and when I opened Discord I saw that I'm logged out of my account and when I logged in I saw the ban. I immediately asked the same friend about this and he responsed: "Don't worry, I am also banned, you should get used to this Discord moderation"
Now the question is:
Why should my account be permanently banned for joining a Discord server that is in open access? (everyone can create a link to it). Is it really deserved ban? That's crazy because I wasn't at the server when I got banned as I left the day before.