Can you download and save the .apk file of the older app perhaps? This obviously would require you to know ahead of time what you were going to potentially use the device for in the future, which is ridiculous.
Yes, you can do that with the adb CLI.
Of course it's a viable solution only for an archivist/advanced users, not so for common users who just wish to keep their old device running smoothly.
We have a Ring with the app installed on an old phone that we use as the "household" phone. I was going to "upgrade" the household phone to be an old tablet which while newer and running a later version of Android, is still old. Can't install the Ring app on the newer tablet because it is too old. So we are stuck with the even older phone unless we buy something new.