ASSTR was a very old site, one of the oldest on the web -- in fact, it's name is "Alt Sex Stories" which is a reference to the Usenet forum, alt.sex.stories, which predated the HTTP protocol websites run on. The usenet group was created in 1992, 7 years before HTTP.
https://en.wikipedia.org/wiki/Alt.sex.stories
The same guy ran ASSTR the entire time as far as I know, Ray Del Sexo. There were volunteers that helped moderate the moderated version of the usenet group, but they stopped in the mid 2010s.
It's highly likely Ray passed away without setting up a successor plan. That's the theory I've heard repeatedly. He went off the grid and no one could get in touch with him for many years before the site finally died. It's likely he passed on and the business account just kept running for a while until it ran out of funds. Even then, the site basically was unusable since about... 2012? I think? The search engine was broke, users couldn't register, etc.
Sadly, a troll earlier this year had the Wayback machine exclude the ENTIRE site from the Internet Archive. They are not the free speech warriors they think they are; they have capitulated in many times over the past 10 years to censor websites and news content that was embarrassing for various special interest groups or governments. With the Wayback Machine archive being gone, it's highly, highly likely a huge chunk of Internet Erotica History has been lost forever.
Now, you mention the Forced Nudity Story Archive. I just noticed they're down, too, but they weren't on ASSTR anymore. They were hosting the site on a Netlify site, and they're gone now. It's highly, highly likely someone found their site and complained to the host. There is currently a bit of a moral panic amongst some political types about the kind of erotica that they hosted -- frankly, I half expect them to come for girlspns soon -- and it's all too easy to complain and have people shut down and their websites canceled. It's nearly impossible, thanks to the shrinking of the internet over the past 10 years, to find a web host that won't capitulate immediately to these demands; even if you do they will simply go after that web hosts's payment processor and the web host will capitulate immediately anyway. For an example of this, see StoriesOnline, and the scare they had recently with mass story removal.
The take people have had is that "there's a limit to how much free speech is good" which is a horrific, wrong headed, stupid point of view, but the lack of education in the west is slowly causing the moral decay of our society. No, porn and tinder isn't the dangerous moral decay, "there should be limits to free speech" is. The latter is how you get blaspheme laws and the people pushing this idiotic mentality don't realize it won't be them who gets to decide which heretics get thrown off roofs or burned at the stake.
But I digress.
FNA knew this would happen, they bounced a few times between sites. They have a status page here, with an archive of the site available for download:
https://justpaste.it/fna
If this was a troll harassing them off their host, this will be targeted too.
As they mention on their site, if you know of a web host that will host the kind of stories that FNA and GirlsPNS and IO and the like all would host, let people know. They're vanishingly rare and getting more rare every day. It's almost to the point that people are going to have to move to TOR or something for erotica; some political types already have due to threats from their governments.