We have already seen the iPad Air 2 run Windows 98 and play Fallout 2 like a boss. As if running a decade old OS on a device whose category did not even exist back then was not already enough, Jules from Hacking Jules has managed to get Windows 1.0 up and running on his iPad Air 2.
The first independent version of Windows, Microsoft had released Windows 1.0 back in 1985. Back then, running Windows on a PC required a computer as big as a mini-fridge and a truckload of patience. The iPad Air 2, on the other hand, is thinner than a pencil and can work without being connected to a power source for over 10 hours.
Obviously, getting Windows 1.0 up and running on your iPad Air 2 is not really going to enhance the functionality of the tablet in anyway, but it still makes for a fun experience. Now, keep in mind that Jules has not replaced iOS 9 on his iPad with Windows 1.0. Instead, using DOSPAD, he managed to emulate and get Windows 1.0 up and running on the tablet.
Jules has not detailed the whole process, so you cannot get Windows 1.0 up and running on your iPad Air 2 if you wish to. Nonetheless, the chances of anyone actually wanting to run a two-decade old OS on their iPad seems unlikely to me, except for the pure bragging rights.