Plan 9 makes extensive use of pure-text IPC, more commonly known as file writes and reads. This is facilitated by the fact that each process sees its own filesystem hierarchy.

For instance, GUI programs are built by reading from files like /dev/cons and /dev/mouse and writing to files under /dev/draw. Now it seems that this will end up in a mess when you try to run multiple GUI programs, but that is not the case - a WM can provide virtualized versions of these files, and each program still feels that it is using the entire screen and does not need to know about the WM at all. My description is likely not very accurate because I haven't really tried it, but you can find the whole story in the related man pages: http://man.cat-v.org/plan_9/3/cons http://man.cat-v.org/plan_9/3/mouse http://man.cat-v.org/plan_9/3/draw.

Another example is the authentication facility of plan 9. Instead of being a dynamically-linked library, it is a separate process that can be reached via IPC. Russ Cox et al have a talk on this (https://swtch.com/~rsc/talks/nauth.pdf).

This is also why Plan 9 has no dynamic linking, since those things are done with IPC instead.