How to use older software

Unless the app’s developer updates it to work with iOS 11, you cannot. It is up to developers to update their apps to be iOS 11 64-bit compliant. If they have dropped support for an app or abandoned it, then you cannot use it with iOS 11.

If you upgraded to iOS 11 and you had lots of older, 32-bit apps that would not work with iOS 11 ( iOS 10 put up pop-up warnings that such apps will need to be updated for them to still function under iOS 11) if these apps have NOT been updated in years, then these apps either will no longer be updated and/or have been abandoned by the developer and will no longer ever work under iOS 11.

There is no way to retrieve the data, either.

You would have had to backup and save/retrieve the data in those apps BEFORE you upgraded your iPhone.

IF you have any other iDevices still on iOS 10 or earlier that still have the apps on them along with the data , you can backup save this data to iCloud or iTunes or Dropbox OR even email this data to yourself and find other apps for your upgraded iOS 11 iPhone that you can import the data into, from these older, unsupported apps.

If you have upgraded to iOS 11, you can not downgrade.