I've never really seen it as a FAF-specific problem. It is just another aspect of the very aggressive, divided, tribal culture that is accelerated and encouraged by the internet and social media. Similar to how at this point, Star Wars "fans" are people who go on the internet to create and watch videos hating on the latest Star Wars content that comes out and the people who make it.
Everyone wants to feel like they are smart and better than other people, and it is quick and easy to get that feeling by talking shit about what other people are doing. Joining a tribe that says how stupid and/or malicious others are and implying that they could do it better. Nothing is perfect, so people can always find flaws to point out to justify their toxic behavior. I think many people, particularly the type who spend a lot of time online, don't have very enjoyable or meaningful lives and that makes them more likely to engage in online bullying and tribalism.
I have been quite guilty of this behavior on FAF. I admit that I am an arrogant asshole and probably caused some harm to FAF by being mean to people. I do often feel bad after I write mean posts on the forums, which is why I pretty much checked out of the community entirely and tried to focus on my own projects.
The following paragraphs are intended to be some armchair psychological evaluation of myself and why I do not respect FAF and am mean on the forums. I do not claim this logic is objectively true in any way, or that it necessarily applies to any of the other people who write mean stuff on the forums. Being mean on the forum is less than useless and makes my life, and others' lives, objectively worse.
I have never had a very healthy relationship with FAF. It was just an addiction and coping mechanism for my own miserable and pointless existence. Even when I played every day for years, I never had much respect for the project, and still don't. It's just a video game that offers little more than a way to burn time. On top of that, nobody here even created the game. I never really cared, and still don't care, if FAF dies.
A big difference between FAF and the other games mentioned in the thread is that those games were actually made by the people who are changing them. It is their creation and they have the right to do whatever they want with it. There is reason to believe that the changes they make will be good because they come from the same people who made the game you chose to play in the first place. On FAF every change comes from some random amateur who appointed themselves the authority to change whatever they want without earning the right to do so. So I think it is reasonable for FAF changes to be met with more contention than normal games.
In response to criticism, these people will usually point out how they are unpaid volunteers keeping the game alive so we can all play it. It comes across as an attempt to gain some moral superiority as if perpetuating some useless videogame someone else made two decades ago is some kind of charitable cause that benefits humanity. Just because I waste my time playing this game, rather than some other game, doesn't mean I owe you anything. When making a contribution, you take on the responsibility for whatever happens as a result of that contribution. It doesn't matter if you were paid for it or not. This attempt to gain superiority and avoid responsibility only encourages further hostilities. I think it would be better to either say nothing or admit some responsibility which would then generate sympathy and make further aggression more difficult.
These points are even more true for moderators since they don't even have to have some coding skills or anything to contribute. They just get power handed down to them from the beginning of time that grants them the right to judge players and make up random new rules. I haven't personally had many conflicts with moderators though.
I can't comment on anything that's happened in the last two years since I haven't been participating in the community. I left as a result of my self-induced suffering, not because of any FAF issues or changes that were made.
I have noticed that there are almost zero casts of 1v1 games on YouTube anymore, outside of major tournaments, which are also quite rare now. It seemed to me that the focus of the community shifted away from 1v1 once mapgen and TMM became things. I also disagree with the ever-increasing shift for 1v1 games to be played on larger and more complicated maps. These factors mean I will never return to FAF since I play almost exclusively high-rated 1v1 games. Though my return is unlikely regardless. So those of you who dislike my toxicity can celebrate, and the few people who keep trying to get me to play 1v1 can be sad.