Usenet was another source of information in the 1980s 1990s that started in pre-www and started dying off by the late 1990s as the internet was more and more commercial in nature.
“Google Groups” has the largest repository of Usenet archives although there are backups all over the world. They date back to the early 1980s and it’s still in active use in its odd decentralized form.
But then you get online forums/ message boards. As those are run by individuals, they get wiped out constantly and it’s annoying.
And apps? Oh so much stuff hiding in apps that come and go, no archives.
Websites are also often run by individuals and they come and go. Sometimes by companies. Wikipedia’s longevity amazes me but it could vanish tomorrow.
But all of this vanishing data is primarily accidental or “corporate”.
To go through and actively remove data from every computer is no easy task:
Example: abuse. why do they have such a hard time removing it all? you’d think they’d get rid of it all. and they try. but they can’t.