You can transfer the apps from iTunes even if they are no longer available. You can even keep old versions if you copy the .app files or rescue them from the re

You can transfer the apps from iTunes even if they are no longer available. You can even keep old versions if you copy the .app files or rescue them from the recycle bin after the updated version is available. I suspect you can only install them on devices attached to the correct account and I'm not sure what happens with the Mac App Store (or Google Play, Windows store etc.)