I think the biggest issue, what lead to the toxicity, came down to the question/answer format not suiting the problem it was trying to solve — The answer could only be as good as the original question, and the platform gave little leeway to "get to the bottom" of the problem. Getting to a high-quality question/response required a back-and-forth that the platform made difficult by burying the discovery/definition work in comments and edits instead of a clear discussion mechanism.

All of this meant the learning-curve on how to participate was high, and this spurred gate-keeping and over-zealous moderation. High-quality but out-of-date information was preferred over lower-quality but more recent updates. When combined with the rapid shifts brought on with mobile development and web frameworks, the answers could easily get out-of-date months after being answered.

I remember a time when StackOverflow dominated every search query. Now we're seeing searches take you to a dedicated forum/discussion board, which feels more appropriate for the current state of the industry.