And riffing off the fact that the previous Bell Labs Unix was version 8.
[1]: http://doc.cat-v.org/plan_9/1st_edition/designing_plan_9
The abstract notes that it was published in DaemonNews in 1999.
I tracked down the DaemonNews publication: https://web.archive.org/web/20000819035036/http://www.daemon...
The copy in DaemonNews includes citations that were elided from the copies on cat-v.org and darwinsys.com. Notably, we see that line change from
> (More recently, much of this work has been consolidated into an operating system called Plan 9 From Bell Laboratories.)
to
> (More recently, much of this work has been consolidated into an operating system called Plan 9 From Bell Laboratories.)[Pike1990a, Pike1995a]
However I think plan9 started as a skunkworks project well before Unix v9 was released.
[edit - wikipedia maybe supports me ]
> Plan 9 from Bell Labs was originally developed, starting in the mid-1980s,
For instance:
> Plan 9 from Bell Labs is a research system developed at Bell Labs starting in the late 1980s.
https://9p.io/plan9/about.html
The Wikipedia's citation on the sentence saying "mid-1980s" is the press release for the 2nd edition of Plan 9 (1995, the first generally available version), which doesn't mention anything of the sort. That is to say: Wikipedia's "mid-1980s" claim isn't well-attributed.
(The 1st edition of Plan 9 (1992) wasn't generally available, and was only made available to universities.)
(My above 1987 citation states that they worked on it in 1987 and implies, but does not explicitly state, that there was no work before that.)