Is this actually new or just a rerelease/relicense? I assumed they always released the source code for Microsoft Basic but maybe not. At least they did for the 6809 version https://colorcomputerarchive.com/repo/Documents/Books/Unrave...

Those are all disassemblies, not the original code (although the original code for 6809 BASIC was recently found, see https://github.com/davidlinsley/DragonBasic). Similarly, the blog post links a version of 6502 BASIC by Michael Steil on Github that's set up to build the BASIC ROMs for a variety of computers, but that appears to be based on a disassembly rather than the original code as well. Here's a random bit of code (the code handling processing a FOR statement) from Microsoft's repo, vs. the same code from Steil's repo: https://github.com/microsoft/BASIC-M6502/blob/main/m6502.asm... https://github.com/mist64/msbasic/blob/master/flow1.s#L3

I think it was released in scanned form from a printed document (or maybe that was 8080 BASIC), this is the first I've seen in digital text form.