Following Broken Links
Even though, as Tim Berners-Lee says, "cool URLs don't change", it's an inevitability that, even in the world of web specifications, sometimes links break (due to refactors, obsoletion, or just plain negligence). Here are some tips on what you can do when that happens:
Start removing elements of the URL, separated by slashes, from right to left, and see if you eventually find a root with the new structure from which you can navigate to wherever the content moved.
Start removing elements of the URL that refer to revisions, versions, or dates and see if you eventually find a canonical document or redirect.
Try plugging the URL into Google or DuckDuckGo and seeing if that tells you where the content went. (If you know the title of the page you were going to, you can also try searching for that.)
You can try the Wayback Machine at https://archive.org/ and see if it has a cached version of the page.
If you've landed on a page that doesn't appear to specifically pertain to what you're looking for, you can look within that page for instances of a specific term with Ctrl+F in all the major desktop browsers. (Mobile browsers also usually have a "Find within page" function accessible from the app's menu.)