so when a developer removes their older versions of their app from the App Store so that you cannot downgrade to them where do they go? for example if an app requires iOS 10 or later and a developer unticks the older versions of their app so that people on lets say iOS 8 cannot download the latest compatible version, what happens? thanks all :)
The older versions do not "go" anywhere as such. If you're hoping that there would be some kind of "Apple Archives" web site where Apple offers up downloads of older versions that the developer has requested no longer to be used - then you will be disappointed.
When a developer unticks older versions of an app, what happens is that Apple no longer offers that version for redownload from iTunes in the Cloud (i.e. from App Store on a device, iTunes on a computer, etc).
Note that developer often have good reasons for unticking a version. In the end most developers want as many users as possible using their app. It could be for example that an older version no longers work with newer versions of their server software, or that they found a grave bug.