The thing that killed it was the thing that made it prominent.
According to a blogpost i can't find any more about "how to speedrun social media" or similar, it's basically impossible to pivot a community once established given the levers of voting mechanics and mod policy. (I distinguish between Facebook's and Reddit's enshittification and a pivot.)
IOW, the seed of a community property's growth is also the seed of its decline. Analagously, the charming village is attractive to new residents because the existing residents resist the changes that new residents embody.
Very broadly and by no means universally: techies don't like lusers, because lusers don't put effort into understanding, they just want turnkey answers. This is totally counter to the archetypal techie value system.
SO optimised for grumpy misanthropes over bumbling newbies, which, in tech, is roughly congruent with optimising for people with knowledge to share over people who want their homework done for them.
The questions are not the scarce resource - you can browse Microsoft Community Answers to verify that. The answers there are correspondingly useless, which is why "blindly copy from SO" is a meme and "blindly copy from Microsoft Answers" is not.
Here ITT, and more generally on HN, comments are excoriating about the cost of being offputting to new users. Those commenters should recognise that it's not a pure cost, it's a tradeoff, and the upside has been a treasure for a decade. A million projects completed by people who found quality answers, mostly by searching for existing questions.
That resource is still relevant for stacks other than the latest web framework hotness, and i personally still get benefit from it in my archaic hobby of learning to code.
That resource is not still directly relevant for coders deeply plugged into AI, but it did train that AI.
It's one of the most successful endeavours on the web and a towering community achievement. It is a superficially a shame that people experienced gatekeeping, but if "gatekeeper" has only negative connotations in your lexicon, then I advise you to compromise your principles and firewall your network.