A lot of people still read the Bible and other religious texts outside of English class.

People still read a lot of classics.

Tons of legal documents, laws that haven't changed, and other bureaucratic documentation is still referenced regularly.

Pretty sure there are architecture plans for things like the Hoover Dam that are still used for making maintenance decisions.

Historians sure look back at old documents, not to mention people who do genealogy and other family history stuff.

While the average person may not actively seek out old text because new content on the internet is the current hotness, there is still a ton of information that people who are working jobs or doing research need that is over a century old and that is the kind of information that is comparable to something like stack overflow. It will more likely be copied and retained through something like archive.org than the original website, but as long as there is a chance that someone might need it or someone has retaining it as a hobby, it will still be around.

Given some software is still dependent on stuff written like 20 years ago, it wouldn't surprise me if even a small part of some future program relies on century old code