Is it a common practice for Discord to enter in contractual obligations with businesses that use Discord as a central support or distribution channel? I've never heard of this before and given what happened to OP it seems like this should be (a) common knowledge that you can do this and (b) a default option for businesses to consider, if Discord is indeed a big channel for your business.

We use Discord as a primary support and community channel and it never crossed our minds to make a contract with them, but now I'm thinking about it..

I am quite confident that it was this activity. It is the only thing that fit this description:

"Your account maliciously shared or participated in the sharing of the personal or private information of another user."

It was also the only message of mine that was deleted in the past week.

Partnered servers are definitely not. There was a kneejerk moderation action taking in the past to ban the official FFXIV server after a user posted some illegal content there, reported their own illegal content, and deleted the message afterwards so mods couldn't locate it. Considering where Discord came from, it was a surprising move to not even ask the operators of the server or share any information until they were facing user pitchforks.