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

.... Maps?

Plat maps, land use maps, survey maps....etc can date to the early 1900's and late 1800's. Preserved & converted to digital form for country-side USGIS visualizations and mapping.

Or architectural blueprints, or sewer/water line diagrams/maps. Electrical & communication lines are approaching that age. Transportation infrastructure and diagrams.

We reference documents & technical knowledge that's 30-50 years old, sometimes 70-80 years old right now in our industry. Mathematics knowledge that's well over 100 years old.

There are countless examples of important things that are as old as the industry itself.

It's essentially guaranteed that we will be referencing technical documentation and design and knowledge that is over 100 years old once this industry is old enough.