You are not alone. I like to read Harry Potter fan fiction [1] and I have started checking the publication date when Im searching for something new to read. I started doing this passively and realized it after the fact.
Have you ever met someone who could say all and do the right things but never made you feel anything, or your gut was sensing an ulterior motive? It's a magic trick we are all bewitched by at some point in our lives. I suppose I filter by published year because I dont want think about if I am being tricked or not.
[1] There are some very talented writers[A] out there who (I assume) cannot do the world building part.
With Harry Potter (and Star Trek, I believe) fanfiction, I think decline in quality has been typical for the last decade; the fanfiction from the 90s/early 00s is often of much better quality on average just due to the age + other factors affecting who was able to type up and post fanfiction online back then.
There are people churning out 40k word novels daily on AO3, and they get the eyes and feedback that up and coming writers desperately want. Real content is being drowned out and ppl are hurting because of it.
I've spent very little time on ao3, but I imagine it's slightly better than RR where seemingly a mark of value is "huge length (word count and chapters)" with weekly updates.
I left a few creative writing forums I had been a member of for over a decade because of how much garbage was being posted. Perhaps even more upsetting than the number of people flooding the front page with reams of nonsensical AI prose was the number of people who saw nothing wrong with this and, instead, heaped praise on it and enthusiastically cried for more of it.
It was profoundly alienating to see hundreds of people liking and commenting on chapters of obviously AI-generated prose full of LLM-isms and broken metaphors and similes - and these 'authors' have the gall to link a patreon account and ask for money for this tripe!
We had a number of ways of detecting sockpuppets, and I was part of the mod team at the time, so I could verify they were real users. The change in standards was a combination of things; the old guard, many of whom had left at that point, leaned towards computer literate STEM students who had grown up reading actual novels. In contrast, the new generation of posters was full of ESLs, nontechnical users, and people who grew up reading stories on FFN and RR; the new users were also unfamiliar with forum etiquette and acted like they were on discord. Suddenly, I was having to remind users that English was the official language of the board.
(The phone-vs-computer element also can't be discounted - it's much, much harder to do in-depth long form discussion on a smartphone. Input is a hassle, and doing a bunch of research is extremely annoying when you can't open multiple windows and an editor.)
After 2023 or so, a new problem popped up: posters would read a chapter, hallucinate a series of events that never happened, get mad about what they thought they had read, and then attack the author! Arguments born from a complete lack of reading comprehension were breaking out in all the busiest story threads, with waves of reports every time a new chapter was published. It would not be an exaggeration to say that the average IQ of the board had dropped 20 points.
There was also the monetization aspect to consider; fanfiction used to be by fans, for fans - the work of amateurs, in every sense of the word (amātōrem, 'lover'). While that certainly didn't mean everything was /good/, it at least meant it was a work born of genuine passion. The new authors treated it like a business, churning out updates every week and advertising to readers that they could read several chapters ahead on their patreon. Without even touching the ethics of the matter, you can imagine how the incentives changed the output.
Have you ever met someone who could say all and do the right things but never made you feel anything, or your gut was sensing an ulterior motive? It's a magic trick we are all bewitched by at some point in our lives. I suppose I filter by published year because I dont want think about if I am being tricked or not.
[1] There are some very talented writers[A] out there who (I assume) cannot do the world building part.
[A] Recent Favorite: https://archiveofourown.org/works/1134255/chapters/2292768