A lot of Plan 9 innovations have appeared in Linux. One of the outstanding issues are union aka. overlay filesystems, and these are currently being addressed.

A lot of Plan 9 innovations have appeared in Linux. One of the outstanding issues are union aka. overlay filesystems, and these are currently being addressed. That said, Plan 9 seems to see continued use in some niches. For example, Plan 9 has been ported to Blue Gene. (What's kinda cute is that the Plan 9 kernel (including TCP and such) has fewer lines of code than the Ruby parser, 8000 vs 10000, IIRC.)