The Atari 400/800 computers had available as an extra cost purchase (IIRC about $20 in 1984 dollars) a "technical reference manual" that included full schematics for the computers, a printout of the 6502 assembly for the ROM OS, and a lot of narrative describing technical aspects of the computers.

So for at least Atari, and at least with the 400/800 computers, they did release the information. For an extra cost, but it was released.