I am sorry to hear you had a post hidden, and even more sorry to hear that it has had such a negative impact. I don't know what caused your post to meet that fate, but I know from experience that it feels pretty awful.
I have some experience with seeing the kinds of things that get a post hidden from serving on MIRT and from voting on juries, so I can share some generalities about how the system works. Apologies in advance if any of the following is stuff you already know.
Generally speaking, any DU member can alert on a post. It can seem pretty random sometimes, because, well, it is random! However, all any individual can do is alert on a post. Once a post is alerted on, the system automatically selects a certain number of DU members who have indicated their willingness to serve on juries and asks them if they are able to serve at that time. When someone agrees to serve, they are shown the text of the post (with all identifying information stripped away), along with the rule from the TOS pertaining to it, and asked to vote on whether they think the rule was broken. When a sufficient number of the anonymously selected jurors have voted, the post is either hidden or allowed to stand, according to how the majority voted.
Most of us have probably had posts alerted on at one time or another without even kniwing it. If one of our posts is alerted on but not hidden, we'll probably never hear about it. None of us knows whether we've been alerted on, unless and until we get a post hidden. When that happens, we get a message notifying us and indicating a reason why the jury voted to remove it.There is also an appeal process.
It may seem like a weird system, and it is definitely not perfect. But I like it better than a lot of the possible alternatives. The founders of this site wanted to have some guard rails to maintain a civil atmosphere, because a message board without those can easily turn it to a free for all, with people fighting and attacking each other, and political issues provide lots of fodder for STRONG disagreements. I don't know what your experience has been, but before I came here, I had been a member of a political forum where people started turning on each other savagely during the 2016 election, and I was one of those who was attacked (because I supported Hillary rather than Bernie). It was...pretty horrible. DU's founders had witnessed that kind of thing, and they wanted to keep it from happening here.
I've come to like the jury system here, with all its flaws, because it keeps things as impersonal and anonymous as possible. I'd actually feel a lot less safe here without it. On some boards there are moderators who make and enforce whatever rules there are, and that can feel uncomfortably personal at times. It also means there are a few people who literally have power over everyone else. I think our alert/jury system is actually a lot more, dare I say it, democratic!
The first advice I would give anyone who has a post hidden is to please try not to take it personally! I know it can feel that way, but it's really not. The jurors who voted on it didn't know who made the post. All they had to go on was some words on a screen. I've served on a lot of juries, and it can be tough. It's not always easy to decide, and the system gives jurors the option to nope out if they feel like they really can't decide for some reason.
One thing I've learned is that people don't always alert on a post for reasons I agree with. I've voted on posts where I felt the person who alerted probably misunderstood the rule or overreacted to the post's content (in which case, I typically vote to let it stand). I've also seen posts the I thought were highly problematic allowed to stand, and posts that I personally would have let stand get removed. Like I said, the system is not perfect. But no system of moderation is perfect, and after participating in this one for about 5 years now, I have honestly been impressed by how well it works, most of the time.
I hope the above gave you a glimpse of the ins and outs of the process and its strengths as well as its flaws. To figure out why a particular post met that date, I would suggest reading the rules in the terms of service (link at the bottom of every page in the site) and see how the reason given for hiding the post lines up with the post's contents. If you feel strongly that your post did not violate the rule, you do have the right to appeal!
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Note: nothing in this post should be interpreted as an attempt to interfere with forum moderation. It is provided as general information on how the alert process works and how it relates to the jury system and the removal of a post.