Thank goodness. I know of no way to support all the libraries in DOS. I also don't want to extend support further back than is reasonable. I feel 'XP or newer' would be fitting, but technically we can still support ME or newer.
Wait people still use Windows ME?
Dear me, I sure hope they don't. It was an awful bag of sick. I'm not sure if supporting it is really any different to supporting XP though. Then again, I barely remember anything about ME: I used Win 2000 Enterprise when all that rubbish was rolling off the line.
Pretty much w2K, XP, and Win7 are the only decent MS OSes since Dos 5 + Windows 3--and I was big into OS/2 at that time.
I should note, that I was very impressed by the authorof Shovel Knight. Making a version in 80386 assembly, that runs on a 386SX-16 (maybe -25) in DOS is rather incredible.