Well, considering the last DOS version is 6.22 (or, if you stretch, Windows ME)... by virtue of running DOS, you're running old software almost by definition :) Yes, I know FreeDOS exists.
There were a few versions of DOS from other vendors available after MS-DOS for example IBM PC-DOS and DR/Novell/Caldera (same OS with name changes as the owner changed) DOS.
FWIW, OS/2 is actively developed, maintained for a price. I gather it's mostly for industrial control and ATMs these days.
I used OS/2 Warp 4.0 for a bit, when researching for my book on Windows NT. It's certainly not "DOS", or not only, as it's a protected memory kernel with DOS userspace. But it might be a DOS, or a direct descendant that has managed to swim all the way to 2022.
