At a previous job, I had to build laptops that would multi-boot DOS, Win95, Win98, and XP, all on one laptop. Our field-tech’s software had to have those specific versions of windows for the machines that they serviced, because of how the software interacted with USB and embeded floppy drives.

If you want to scare yourself:

spoiler

They serviced Medical devices: X-rays, MRIs, CT-Scanners, etc.