Adding to this, I believe the first C compiler written outside Bell Labs ran on ITS.
The story goes, Alan Snyder was an MIT student interning at Bell Labs in 1973. At this point in time, B was being transformed into C, and Snyder was involved in that process. He’s credited with having suggested the || and && operators and the preprocessor. When he got back to MIT he implemented a C compiler with PDP-10, PDP11, and Honeywell 6000 backends. His thesis “A Portable Compiler for the Language C” is from 1975: https://apps.dtic.mil/sti/citations/ADA010218