Lots of things out there say that Styx was "a variant of the 9P protocol", but I haven't found anything that says what was actually different about it.
- The `typ` values that specify the message type are different (9P1 values start counting at 50, Styx starts counting at 0)
- Tcwalk/Rcwalk have been removed
- There is no authentication (so Tsession/Rsession have been removed, and Tattach/Rattach have been shortened)
There you go.