You can very easily install older versions of software. If you make a local iTunes backup, you can keep the apps and reinstall them locally. If you have an older device that doesn't support the newest version of the app, you will be prompted that there is an older version available that us compatible and be allowed to download it.
The former option works on all iOS devices ever made, the latter works at least as far back as iOS 5 devices.
You can't restore local copies of apps that you backed up using iTunes in iOS 9? But you can download older copies of apps on a first gen iPad? That's weird if that's what you are saying. I was able to download older apps on my 1st gen iPad about a month ago.
Edit:
Are you sure it stopped working? You can't download an older version of an app on a non supported OS unless you either previously downloaded a version before on a compatible device or used iTunes and bought the app on a computer.
Edit 2:
Apparently you still can download the app to your computer directly using iTunes but you can't back it up to your device.
The former option works on all iOS devices ever made, the latter works at least as far back as iOS 5 devices.