Also worth noting that you can conserve old, maybe unsupported and unavailable, apps by downloading them into the iTunes Library in the computer where the program is installed and your iGadgets have a trust relationship.
In Ye Olde Days, when you synced a device on iTunes, the process would download a copy of all non-default apps onto the iTunes Library on the computer for backup purposes. And in the Updates section in iTunes, you could update all your apps in one fell swoop so they would always be up to date. Afterwards you could always reinstall from that copy even if the app was no longer available in the Store. Or when you synced the device again, all the new apps would auto-update from the computer without gobbling up your cellular data cap. Happy. Automatic. Carefree.
Then 'app-thinning' appeared: an app could be 32 or 64-bits and only the version appropriate to the device would get installed from the Store. Apps ceased to be copied/backed up to the iTunes Library on the computer when syncing. No Happy. No more.
But... You can manually go search for each app in iTunes in the computer and download explicitly the 'fat' installer onto its iTunes Library. There you can also update all as new versions come out, like before. And when you sync, they get updated on the iGadgets as well, as before. And you still get to keep a local copy of apps that may become unavailable at the store. Happiness restored. Even if a bit more cumbersome.