i definitely agree that this is a shift we've seen over the past decade or so, but i don't think it's a surprise.
in web 1.0, there was an understanding that the internet was full of different nooks and crannies, and that each community had its own culture, norms, and expectations. the extent to which there was no centralization at all is something that i don't think it's possible to convey to people who weren't there. the internet was truly a web of hundreds of thousands of different distinct communities and sites, ranging from people's personal sites (angelfire! geocities!) to forums with thousands and thousands of users.
most people understood this and this was why being told to lurk moar was such a stinging thing--it meant you had ignored this reality and hadn't invested the time to get to know the community's norms before you started to interact with that community. you were being entitled and that wasn't okay. (sometimes communities took this too far and became genuinely cruel to outsiders. not excusing that.)
of the many, many things that changed with web 2.0, one of the most important is consolidation. social media was designed from the outset to appeal to as many people as possible, to create standardized experiences for everyone, to kind of flatten out the beautiful varied landscape of the internet into a perfectly flat plain/plane (both!). first facebook, then twitter, then instagram, then tiktok (and the smaller platforms scattered throughout) wanted to be all things to all people so as to maximize eyeballs so as to maximize ad revenue. it is impossible to overstate how this changed the internet.
ao3 was never supposed to be part of the great internet consolidation. it was truly "our" own--the project of a small group of dedicated fans who wanted to build a place to post fic that wouldn't be at the mercy of a corporation, all of us having learned the hard way that we could not trust those who were trying to make money off of us. having been around back then, getting my own account in 2009, and watching as friends-of-friends started the whole project, i think it is safe to say that NO ONE involved in ao3 in the beginning thought that it would take off the way that it did. the scale it's on now was unfathomable then.
but due to several factors--a fan-run fanfic depository was a genuinely great idea, the timing of it being created when the whole internet was shifting towards the consolidation model, probably some other things that aren't immediately coming to mind for me--it started to attract people who didn't understand its context at all.
throughout the 2010s and especially during/after (for certain definitions of "after") covid, many new people who had no experience with web 1.0 were coming into fandom. their understanding of what the internet is was shaped by social media and its all-things-to-all-people, designed-and-run-by-giant-companies culture, a culture that is in direct conflict with the diy, esoteric, gift economy, hobbyist, we-built-this-ourselves-for-us ethos of fandom.
these people brought their expectations of what the internet is with them, and it's caused so many problems, y'all. they view fanfiction not as a joyful, playful, collaborative exchange of creativity and mutual passion but as...content.
and content is, at the end of the day, just a capitalist product. it should appeal to the consumers (and to as many consumers as possible), not be seen as individual self-expression or community sharing/exchange. if the consumer doesn't like it, it's failed in its objective. content exists to be consumed and requires nothing of the the consumer except to experience it. "attention economy." if you want to comment or like or subscribe, that's great, but it's not necessary. it's always a bonus, never an expectation, as is clear from the way that "content creators" have always had to beg people to comment/like/subscribe.
the influx of new people view fanfiction and all fannish creation/activities/communities the same way they view "content." this creates an entitlement (if it doesn't appeal to me specifically, it's not doing it job, because content is supposed to appeal to EVERYONE) that is creating friction in fannish communities. it's why people don't comment anymore. it's why people go to discord to talk to their friends about that fic they read instead of TELLING THE PERSON WHO WROTE THE FIC how much they liked it.
and the "don't like it, don't read it" attitude we've all always had simply does not jibe with a web 2.0 approach. the expectation that the internet is a place with infinite variety in infinite combinations and that it is your job as an individual to search out the things that work for you and ignore the rest...has dried up. instead, people are wanting to consume fannish creations instead of engage with them. and when something isn't for them, instead of moving on, they feel entitled enough to complain about it and sometimes even harass the creator.
so you've got the old fans, begging these people to just heed the tags and move on when they don't like something. but these new people have never had to do that before, and so they're lashing out at people who create things they don't like.
so here's where i go from talking to all the people who are like "wtf why are people acting like this?" to the people who are acting like this.
the number one thing to keep in mind when entering fandom (beyond "be kind" which should be the number one thing to keep in mind in any community ever) is: ymmv. your mileage may vary.
different people have different experiences/preferences/likes. different people engage with different things in different ways. your job as a person old enough to be free-range on the internet is to find what works for you and ignore the rest. that's it.
there are a few exceptions to this. if you see something that is genuinely racist or homophobic or misogynist or whatever, it can be a very good thing to try to engage people in conversation about that in hopes of pushing back against it. i'm not telling you never to do that.
but 98% of the time, if it's not for you...it's not for you. and it's your duty to just move on. hit the back button. close the tab. if it really irks you, privately message a friend to kvetch about it. but stop expecting everything to cater to you. let go of that entitlement. remember that fandom is about sharing and community-building and joy. if it's not bringing you joy, go find something else that does. or create it yourself.
fanfiction, fannish works, and fandom in general do not exist to provide you with content. they're not trying to appeal to as many people as possible but to specific people who happen to share a love for something or a common criticism of something they care about or an interest in a thing or a common perspective. people work hard to create things for which they will never, ever, ever be financially compensated. you are not "paying" for content via your eyeballs/attention. you are not at a restaurant ordering off a menu and exchanging money for the food you receive.
you're at a potluck. and if you're at a potluck, it's an asshole move to criticize the dishes people have brought. you pick what looks good to you, you bypass the rest. you are in charge of your own plate.
maybe you try something that looked good and find that it wasn't. so you...don't eat it. you eat something else instead.
if you find yourself in a situation in which, say, you've got celiac's and there are no gluten free offerings, it's legit to be like, "hey, wtf is up with the lack of gluten-free offerings?" but even then, you're commenting on the whole spread, not on individual dishes. you don't get to turn to someone who mixed and kneaded and rose their beautiful bread and get angry at them for not catering to you. commenting on trends is generally an okay thing to do (as poor gen and femslash fans have been doing since pre-internet days). criticizing individual fanworks is not.
any time you see something that pisses you off (that isn't actually racist/queerphobic/misogynist/whatever), take a deep breath, say, "there's plenty of other dishes, i will not go hungry," and move on!!!