How do I download apps that are no longer in the app store?

If they are no longer in the App store they would not appear in that particular list anymore.

If they appear there, they are still available in the App store.

If the cloud icon appears grayed out however, it means the App cannot run on your current device and as such cannot be downloaded to it.

There are many 32 bit apps still available in the app store for older devices, that cannot be downloaded to devices running iOS 11.

If they are no longer in the App Store, it means the developer of those apps has chosen to no longer support them. Apple notified all App developers months and months in advance of iOS 11 that any app still running in 32 bit architecture had to be updated to 64 bit to run on iOS 11. For a year before iOS 11 was released you got a notice every time you opened one of those apps that unless it was updated, it may not work after the new iOS was released. You had plenty of notice as well. Apple has no control over App developers or their apps. They only approve them to be released on the App Store at the then current requirements for apps. The short answer is if they are no longer available on the store, you can't use them.