I attribute much of Stack Overflow's decline to a mismatch of scaling... and that mismatch is in part what lead to the perceived hostility by askers and answerers. Overcorrection and changes (frequently removing of curation tools) often made this worse. (Part of this is inspired by a recent post on meta.stackexchange about how corporate is redoing question deleting and Shog's answer to it
https://meta.stackexchange.com/a/420032 )
The scaling focuses around discoverability, (the rate of) asking questions, and the amount of time available for people to curate (which includes helping new users get onboarded).
Much of the early days of Stack Overflow, discoverability was trivial. It was easy to find an interesting question because few things were asked and this was the days when there was a lot of changes. We're dealing with 2008 and beyond - there were a lot of new frameworks and languages being updated then... Spring 3, Backbone, Knockout, Angular, React, Vue, Spring Boot 1, Java 7 and eventually Java 8.
This was a time of rapid change and the experts were answering on Stack Overflow - and it was scaling up to meet that demand with additional tools.
But it also started becoming a place where students were posting questions (every September you'd get the "how do you draw a triangle with stars?" questions 20 times a day in the Java or Python tags...)
This started being visible in 2010 with question quality and by 2014 it was a full on epidemic.
One part of this is that the original community of people who followed Jeff and Joel's blogs and helped barn raise the site had gotten smaller as an overall portion of the site because of that explosive growth. Some people wanted it to be a place where any user could get help with a question (and closing questions was antithetical to that) and others were looking for it to be more of a wikipedia. That conflict still simmers.
Another part of this is that a smaller and smaller percent of the overall user base was using the curation tools available. Fewer people were closing questions (that really did need to be closed) and helping onboard new users with expectations. When there are 6000 new questions a day that represents a lot of work for the volunteers to try to onboard them. Initially, everyone came from the same point and there wasn't much onboarding needed. With its growth, a lot of new people were coming to it from the approach that it was a forum or immediate feedback chat.
That growth without scaling up the tools for onboarding and curation (making it easier to do) meant that the people trying to help new users and make discovery of well written questions easier had less time per user (or question) to do so... and got increasingly curt - they couldn't spend 5 minutes per question anymore and instead were trying to do things in 30 seconds or less.
This was the beginning of "Stack Overflow is unfriendly."
The attempt that corporate had from this, instead of making it easier to help people took away curation tools and close reasons were made more specific... which in turn meant that the people trying to close questions to make discoverability of well written questions on the home page easier were incredibly pedantic in order to meet those reasons.
Some of the tooling is set up to be able to handle 6000 questions per day, and scales poorly back to having 100 questions per day.
The user base of Stack Overflow at 2 months old is very different than Stack Overflow today, even if it has the same question rate. There are very few people who are around to help onboard users (diamond mods if you read this - there's 25 of you and 100 questions per day... you can spend 5 minutes per new question per day to help onboard the user for expectations and still have half an hour a day for other moderation tasks).
Many of the people who did curation when SO was growing and at its peak have become disappointed with Stack Overflow and while they may look at it to see if its a site where their contributions would again be appreciated... though the changes that corporate appears to want have been additional curtailment of curation tools and have even proposed removing close votes all together ( https://meta.stackoverflow.com/questions/438185/the-company-... ).
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Stack Overflow is mostly dead. It lingers on, but every change that is being attempted or proposed in trying to get more "engagement" takes it further and further from being a place where new knowledge is collected and collated.
I would attribute this mostly to there aren't any curators left - so everything they try building turns into a mess. Corporate has leaned too hard on the community to do curation and diamond mods have largely stopped doing curation in favor of moderation.
With low activity and the largest ratio of staff and diamond mods to active users, this would be an excellent time for those two groups to completely take up the task of curation and onboarding to set a new standard for how it should be done... and if they aren't willing to do it, that really suggests that the final nail in the coffin is in place waiting to get hammered.