Complex sentence: The TOPS-20 operating system by DEC-the second proprietary OS for the PDP-10-preferred by most PDP-10 hackers over TOPS-10 (that is, by those who were not ITS or WAITS partisans).
Split into simpler sentences: The TOPS-20 operating system by DEC was the second proprietary OS for the PDP-10. It was preferred by most PDP-10 hackers over TOPS-10 (at least by those who were not ITS or WAITS partisans).