What is the Foundation?

The Plan 9 Foundation, an Oregon nonprofit corporation, maintains the archives of the Plan 9 operating system software and supports future development by following the practice and processes set forth by the original authors. We strive to continue the work started at Bell Labs in the 1980s. The approach used to model and build distributed systems at that time continues to influence systems to this day.

Source

The sources for the Plan 9 operating system can be found at p9f.org/dl where new releases will be maintained into the future.