What would you have expected them to do? Would you point to the user if Stack Overflow stopped hosting content? Or GitHub? Before places like imgur, I'd see forum postings linking to broken images hosted on personal sites--so self hosting is unfeasible. Geocities data is still around as well as Usenet postings going back to the 80s.

> Stack Overflow

... which licenses its content with CC by SA 4.0[1] and has a public API.

Other licensing models of content will vary.

[1] https://meta.stackexchange.com/questions/333089/stack-exchan...

This is an important point. Someone else is allowed to use the StackOverflow content and likely would if StackOverflow was turned off.

I do think that is worth noting, but it is (mostly) separate from the original point. It doesn't matter if the copyright is open if the content disappears.

At least in the U.S., I don't think it's that relevant to the use-cases I've seen. IANAL, but in the 4-factor fair use test a place like Internet Archive rehosting the content to me easily clears 3 of them if the original host goes away.

These days IPFS is getting closer to being a real alternative.