A couple of weeks ago Mac App Store apps disappeared from the purchase history tab, making re-downloads impossible. It was a bug, Apple fixed it, and purchase histories were restored. Now a similar bug has been fixed on the iOS App Store, and purchase histories there are also being restored.
Just like with the Mac App Store, iPhone and iPad owners began noticing apps and games were missing from their purchase history earlier this week. Since the App Store is often a black box, lots of speculation followed. Fortunately, the simple answer in this case was the correct one.
Not all apps are back yet, but should be as the fix rolls out.
Note: Developers can still remove apps permanently if they choose to, for example if web services shut down or the code no longer works with modern devices. In those cases, when the apps wouldn't work anymore anyway, there's nothing to be done, even by Apple. It's the developer's call.
Otherwise, if you noticed you couldn't re-download some old apps and games, hang tight. They're on their way back!