[gaius]

I dunno, they came enough schematics that you could interface anything you wanted to the user port and read/write to it with what we now call memory mapped I/O. RS232 was a universal standard, the most you would have to do is physically make up a cable. Both the Beeb and the C64 came with a manual that explained to you how to patch the OS (via vectors). On the C64 you could even switch the entire OS out and get its entire 64k of address space, just you and the hardware.

[rwmj]

The original article wants the equivalent of an open source Z80/65xx/68k, fuse settings for all the ULAs, schematics for all custom-designed chips, and open source ROM disassemblies.

1980s home computers didn't come with these, even though they had (for the time) very powerful custom graphics and sound chips (eg. Amiga [1]). Of course, some of it was reverse engineered, and some of it has been released long after the fact.

[1] https://en.wikipedia.org/wiki/Original_Chip_Set

[Someone]

1980s home computers maybe did not, but one cannot not claim Apple was locking you in in 1978. I think ftp://ftp.apple.asimov.net/pub/apple_II/documentation/misc/a2_reference_manual_alt.pdf shipped with every Apple ][. It seems to contain enough information to clone the machine (electronics diagrams, timing diagrams, ROM disassemmbly, etc.)