The Michigan Terminal System was an early interactive time-sharing operating system that ran on IBM 370 (and 367) architecture mainframes. It became obsolete when the services it provided were better provided by a network of computers including personal workstations, file servers, and mail servers. Although a contemporary of Unix, because it's licensing was less open than that of the AT&T and University of California, it never became widely adopted, and supported beyond a few institutions.

MTS was always concerned about privacy and security, as it developed as the main academic computing resource for several world-class universities (and was therefore a target of interest for many computer science and computer engineering students).

Some additional links of interest:

My own involvement with MTS included: