I jailbroke my iPad 4 now and with an app called "coolbooter" I could install iOS 10 in parallel to iOS 7 (dual boot) to see if it's usable. Seems similar to iOS 7. But now that I have found much better displays I truly notice how bad the iPad's screen is for me. It is "usable" but only at super low brightness or the headache kicks in quickly.
I think all 32-bit iDevices, including iPhones, can be jailbroken and downgraded at least in the form of a dual boot. Might need a lot of time to inform oneself, but maybe one of you has old 32-bit iDevices and wants to experiment.