It is possible to download an older version of iOS apps from the App Store.
To do this, you must have already purchased or downloaded the app under your Apple ID. Once you have done this on one device, you can go to your list of purchased apps and see the app; download it from there. You cannot download an older version of the app from main App Store entry.
For example, Graphical Analysis GW is marked in the App Store as being compatible with iOS 11 and newer. You can still get a version of Graphical Analysis compatible with iOS 9 or 10 from the list of purchased apps (but not from the main App Store entry for the application–you must go to your list of purchased apps).
You must have obtained Graphical Analysis GW at some point; if it has never been downloaded under a given Apple ID, you will have to use an iOS 11 device, log in under the desired Apple ID, and get the app. You don’t need to stay logged in after this one-time download.
If you acquired Graphical Analysis in the past, you need do nothing but the next steps.
Now take the iOS device with the older version of iOS, such as iOS 9. Check that the device is logged in to the App store with the same Apple ID that has already been used to acquire Graphical Analysis GW, and log in as needed. Launch the App Store app, and view the list of already-purchased apps. Graphical Analysis will be in that list. Download the app from there. You will not be able to get Graphical Analysis from the main app store entry.