More specifically there are two components to this question - it being OK for the code to be released, and the code itself being available to be released :). I'm specifically asking after the former; if noone knows where the code actually is right now, well, it can be dug out later.
Nice idea, but I honestly don't think it has much value for study. It was a solution to a problem which is no longer important, and what impressed David (and was fun for me) was implementing it under constraints (8086) that are no longer relevant. I would vote for some of the other stuff mentioned by others, like TeX as an example of mastery of both the application requirements with beautiful algorithms and inspirational documentation, PostgreSQL as a thriving large system with brilliant modularization that has enabled research, or LLVM as a pinnacle of code generation which has enabled a golden era of language and hardware design over the last 20 years.