As stack exchange ages old answers frequently lead to websites that don't host the content that they hosted at the time the link was placed. The result is linkrot and answers that aren't as useful as they were when they were written.
I therefore propose that Stackexchange does automatic backups of those websites. By default it shouldn't display the content. Sometimes Archive.org themselves has a copy but they don't store every website. Depending on one central storage is also suboptimal. Google often deletes their caches after a while.
However a keyword [backup] could be edit at the end of a link to switch Stackexchange behavior and show the backup in place of the broken link.
Furthermore Stackexchange could add a flag for broken links. It might even detect them automatically by analysing which pages return 404 errors.