> I also quit when people began to care more about correctness than helping and made my contributions much less valuable ("you cannot post, this question has been deleted").

Same boat here. I also have been a SO user for over 10 years and my account says "top 2%" (though this is entirely from my early participation, before I largely gave up on it). I've gone through spurts of trying to participate again by answering questions over the past 5 years but always come away depressed and demotivated about it.

There's a bunch of things that happen:

* Questions get closed as duplicate incorrectly.

I've tried to argue for the question on a few occasions. One of my attempts was by providing a detailed answer which, by being clearly different from the "duplicate" answer, I hoped would prove the question was distinct. A moderator soon deleted the entire question. That demotivated me from answering anything for a year or so.

Since then, I've only argued in comments the question wasn't duplicate, but basically always get outvoted and the question is closed and eventually deleted. It takes a lot of effort to argue the question should stay open, and it takes basically no effort to click "close" so really, this is what the majority want.

* Questions get closed as duplicate -- correctly -- but the marked duplicate is a very old question and no longer contains the "best" answer.

When there's already a half dozen answers, and the top one has tens or hundreds of votes, it's basically impossible to get visibility to a newer answer. This is so bad that when I'm looking at a question trying to help with something I'm doing myself, if I come across an old question I usually sort by 'newest' to find the best answer. More times than I can count, a recent answer with fewer than 1/10th the votes of the "best" answer is the current correct answer.

Likewise, I have answered these but it takes years on an old question to get even a few upvotes, and it's pretty unfulfilling.

* Poor quality questions

If you try to look for older, unanswered questions (eg, don't want to participate in the "who can answer this new question fastest" game) it's very hard to find decent questions. There really is a lot of garbage that gets asked. My experience is when you do find one, after going through a few pages, you run into the next issue:

* New users don't come back

You answer a question from a new user, and maybe get one vote. The original user doesn't come back and mark your answer as accepted or comment as to why it wouldn't work. No one else provides any feedback about it being bad. It's just "internet points" but it is demotivating to put in the effort and have it go nowhere. Maybe I really am just bad at answering questions (but was good 10 years ago, and the high level position and respect I get at work is a figment of my imagination) but even then I'd prefer to get feedback.

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These are just my experiences and opinions. I'm also not sure how to solve these issues, and I don't think it'll be easy to. Seeing as the rules governing the site are largely made by the community, and the "community" behaviour is largely the cause of many of these, the community is essentially saying they don't see these as problems or want to fix them. Getting past that hurdle is the first step to improving things, and honestly, I have no idea how I can help with that (other than posting long rants like this and maybe helping slightly shift some tiny fraction of people's mindset).

In its early stages, SO was an amazing site to learn from -- both asking and answering questions -- but I think the current policies and behaviours mean this isn't the case anymore. From what I can see, most new users are going to have a terrible time with the site if they try to actively participate. It's a shame.