I'm curious why you asked "when did HN become this" if you're now telling us that it's been "this" for over a decade.
> which has been documented and remarked upon by others on sites
You should supply links so readers can make up their own minds about how convincing this documentation is.
https://news.ycombinator.com/item?id=43141350
These are examples of users complaining about HN in the way many users always complain about HN. You'll find countless such comments making general claims about how the site is "all X" or "has become Y" or "never allows any Z".
The problem is that these perceptions are not reliable—people base them on datapoints that they happen to notice because they dislike them so much [1], and from there they jump to general conclusions.
[1] https://hn.algolia.com/?dateRange=all&page=0&prefix=true&que...
Why this illusion, and how, you may ask? I can tell you why and how! It's because people are far more likely to notice the data points they dislike (https://hn.algolia.com/?dateRange=all&page=0&prefix=true&que...). Not only that, but the dislikeables make a stronger impression when you do notice them. This is why people with strong feelings on a topic always feel like the site is going to the dogs—they're unintentionally blotting out half of the data.
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Edit: here are more recent examples in case helpful (what can I say, it's a hobby)
The HN crowd is far right - https://news.ycombinator.com/item?id=41370117
libertarian tech bro site - https://news.ycombinator.com/item?id=41674836
a haven for alt-right trolls and hateful abusers - https://news.ycombinator.com/item?id=41799329
HN has also been taken over by fascists - https://news.ycombinator.com/item?id=42059688
techno-libertarian pseudo-intellectual fascists - https://news.ycombinator.com/item?id=42393303
HN has gone way to the authoritarian right - https://news.ycombinator.com/item?id=42066014
tows the line when it comes to techno-fascism - https://news.ycombinator.com/item?id=42781203
Hacker News is pretty much far right when it comes to politics - https://news.ycombinator.com/item?id=42618465
I've got just as many pointing the other way, of course, but the valuable examples point opposite to the illusion. That is, if I were replying to one of these (^^^^) commenters, I'd point them to your post instead!
See also
https://news.ycombinator.com/item?id=13110004 (Dec 2016)
https://news.ycombinator.com/item?id=25391526 (Dec 2020)
https://hn.algolia.com/?dateRange=all&page=0&prefix=true&que...