You know that any older apps on older iDevices still function as they always have.
The older app versions still work.
I have the first 2010 iPad loaded up with older versions of the same apps I use on my newer iPads and those files can still be opened between these versions.
Older and newer DropBox, on all my iPads still work to transfer files/data back and forth to my other iDevices and my Mac.
All of my older paid streaming services on my old iPad 1 still work.
Older versions of my favorite drawing apps still work, but I can only use a regular capacitive stylus, the newer, battery powered, "active" fine point stylii OR my fingers to draw as the Bluetooth spec, similar issue with iPad 2, is too old to use any current, modern Bluetooth "smart"stylii.
Older apps on older iDevices DON'T simply stop working, you just don't continue to receive anymore app updates with more new or better features.