At 2023-01-20T14:44:51-0800, ron minnich wrote:
> I am trying to remember when fd 2 (aka stderr) became a thing. I have
> a vague memory that it was post-v6 but that may be way off.

The story goes that it swiftly followed the creation of troff.

https://minnie.tuhs.org/pipermail/tuhs/2013-December/006113.html

Regards,
Branden