Do you have a source for that? Bell Labs Unix had version 9 in 1986 and version 10 in 1989. I suppose it's possible that these happened after Plan 9 was named; but as best as I can tell, work on Plan 9 started in 1987[1]; after V9 Unix had been released.
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.)
[1]: http://doc.cat-v.org/plan_9/1st_edition/designing_plan_9