I’m not sure what’s keeping Microsoft and ibm from Open Sourcing all the rest of the DOS versions as well — the 3.x series was very influential, 5 added disk compression, and 6 was the most modern of them all. I can’t remember if Stac’s lawsuit against Microsoft would require them to take out the disk compression parts (although AFAIK the relevant patents are probably long expired now), but even if that’s the case having these available as OSS would also be useful — even if only for a historical context.