To answer this question, I'm going to take the twelve options from this poll and re-organize them into three new categories. While some of these ideas have overlap, I've tried to put them into the category I think is most relevant to the points I'm going to make.
surveillance state worries [also stated by the poll as: not wanting to be perceived]
social anxieties in an increasingly complex online culture
general state of the world [also stated by the poll as: burnout]
"antis" and other harassment campaigns against creators
creators posting complaints about comments (people worry about commenting wrong)
fandom is walled off from each other [also stated by the poll as: rise of discord popularity]
capitalism turning fandom into a passive "view and move on" commodity
rules from social media impacting fan culture (eg. don't comment on old posts)
tumblr porn ban and other reasons people left the site
COVID/surge in fandom due to quarantine and lockdowns
fandom olds didn't teach the newcomers how to fandom
new entrants to fandom don't know the old ways
Between Privacy and Discovery: How Do We Build Community?
While grappling with this question, I looked over this post titled The Places Fandom Dwells: A Cautionary Tale. Here's a quote from the latter about what it was like being in fandom on LiveJournal in the 2000s:
At the time, LiveJournal was the primary blogging platform for fandom. Its friends list and threaded conversations enabled fans to easily find each other and have discussions. Its privacy settings allowed fans to share as much, or as little, as they chose with others. It was a place to publish and archive fan fic, art, and meta. These features give some idea why the deletions of so many fandom blogs were so devastating.
Notice how the post describes an online platform that gave the user a mix of tools to control privacy and discoverability. Now think about online spaces today. Can you sign up without using your legal name? Does that platform have privacy and moderation options to help you curate your personal or communal experiences? Is there any e-commerce of creative work on that platform and, if so, how does the platform support or restrict buying or selling those works?
In 2008, the Organization for Transformative Works established the open beta for Archive of Our Own, a space which made a point of defining itself primarily as an archive, not an online community. I personally moved most of my fanfiction over that I still cared to have be discovered by other fans and I locked or deleted all other works I had originally posted on LiveJournal and Dreamwidth that were still under my control. When I heard that generative AI bots were scraping fanfic for their learning models, I restricted my works further so only those with logins can still read them.
Tumblr has never been a useful tool for me for building communities the way that I did in the LiveJournal days. There are pros and cons to Tumblr's core system of reblogging, especially when an original post gets edited with an update but that's not the version that keeps circulating, which literally happened on the link I provided above. While I love that I can "keep" a copy of a post on my own personal Tumblr if someone deletes their copy or their account goes inactive or they turn off reblogs, I can't see every branch of a conversation if I didn't create the original post and if I can't see it, I definitely can't moderate it.
There's an interesting thing happening with Tumblr since they created the function to turn off reblogs. People will post screenshots and acknowledge in the text of their post that the person who posted it has turned off reblogs but "I just wanted to have it on my blog". This has had the further interesting development that people have begun posting screenshots of text that is not their own after removing the username from the image. This is fascinating because this is in an effort to respect the user's apparent preference not to be involved in the conversation but it obscures the author from the 'permanent' record which may or may not be important information over time.
I believe in the Right to be forgotten but I also care about maintaining records and archives as a matter of accountability and community moderation. So while I was fine with people taking screenshots to make use of existing tools of removing the original account from the conversation at their 'request' (reblogs turned off), I've found I'm actually bothered by what seems to be the 'polite' convention of anonymizing posts so that the re-poster can prove they aren't stealing someone else's ideas or words... but not offering attribution, seemingly at the author's request.
So what's the answer? I'm genuinely not sure! But I would rather copy and paste text with attribution than use a screenshot without alt text, especially an altered screenshot. I don't think people who turn off reblogs shouldn't use Tumblr; the platform gave them a privacy function and it's their right to use it, I can engage with or ignore such posts as I see fit. But online norms already support a culture of screenshots with or without attribution so I can't say that it's surprising that even if the author exits a conversation, their words can continue to circulate online, with or without documentation.
And now I'm going to bring this all back to this poll's starting premise: is (online) fandom "quieter" now than it used to be? This seems to be a question that's actually asking, "Why are online fandom communities less active than they used to be?" and here is my answer:
The digital place that is the internet continuously changes because of "real world" physical, social, and political forces.
The increasing complexity of offline and online life has lead to the greater diversity of human social groups, with some groups more or less successfully growing and maintaining active membership.
Even as people have greater freedom find and join communities, any place where humans congregate (digital or physical) needs rules and practices to address unacceptable behaviors.
Community is what we make it. If we make it on a digital platform that isn't aligned with with our values, we might suddenly find ourselves without a place to congregate. A lot of hay is made over the 'walled gardens' of Discord but I personally don't care that much because living through LiveJournal taught me that no digital place is eternal. There are only records and archives and the people who I've stayed in touch with as the digital landscape has changed underneath my feet.
When I locked my Ao3 works to registered users, I knew I was giving up discoverability in favor of privacy, so I've honestly been surprised to see as many kudos and even comments on old works as I still get. I respond to comments because I like thanking readers but I no longer expect the LiveJournal experience where we friend each other's accounts and I get a feed of fanworks mixed with personal blogs that gives me a chance to know the reader better and build a community with them. I miss that sometimes but mostly I'm just glad that Ao3 exists.
Even now as I try to take memorizingthedigitsofpi's invitation to feel free to add my thoughts in notes on their original post, I feel a sense of impropriety writing such a lengthy essay and connecting it to someone else's questions. I wonder if my words will be reblogged or screenshot or quoted without context but it's a passing anxiety. I have been putting words on the internet a long time after all. It's up to others whether they attribute them to me or if they want to talk to me about them.
Community is what we make it. In the public square or the private gardens, when we share with others, we are practicing vulnerability. It's not always safe and it's not always easy but I hope we keep doing it because the human endeavor to connect over creativity makes up some of the most meaningful threads of my life's tapestry. I hope we make better monuments and libraries, better public squares, better walled gardens so whether I walk the loud bustle of the shared streets or sit with the quieter birdsong of my personal garden I feel those threads holding me in connection with all of humanity. I hope you've connected with me.