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Microsoft releases MS-DOS 4 source code on GitHub — 45 year old code now open-source
(www.tomshardware.com)
Discussions on vintage and retrocomputing
I wonder if they licensed the source of 5.0+ to someone and are still getting paid for it. If so, it's probably something ubiquitous and critical that nobody would think of like traffic lights or water treatment plants.
I think that is likely since 3.3 wasn't included either and that is one of those versions people stuck with for ages.