Nobody is writing Windows or Mac software anymore.

That is a completely ridiculous statement with no basis in reality.

When you search for QR scanning software there are 45 options for iOS, and only 2 for MacOS one of which is an Air app and the other costs $30 and is proprietary and they both suck.

That says very little about whether people are still writing software for Windows and Macs in general and quite a lot about the use case of QR codes for mobile devices vs desktops.