3. Sites or Contents Disappear
Sites cease to exist, change their content, or merge the content with other sites. And once this is done, some links are removed, not deliberately though. And if you have any of such sites or contents linking in, your links disappear too.
You have a very little say, if any, about this kind of situation. It's like driving in your car and you notice a drunk or a reckless driver. You cannot drive your car and the next driver's car at the same time. The best you can do is get out of the way of that driver.
The internet is constantly changing, with old sites going off the grid and newer sites springing up every now and then.
So you've got to expect these situations to happen from time to time. But this shouldn't prevent or deter you from building up your link portfolio.
And in cases where you can trace the new sites that the old ones have evolved into, don't hesitate to reach out to them and request for the same linking privilege.