You are thinking of the Unix compatibility subsystem of z/OS (formerly known as OS/390, and before that as MVS). That came out around 1993. But prior to that there was a long history of running Unix on IBM (and IBM-compatible) mainframes as an independent operating system. This work at AT&T, I don't even think it was the first port, I think Princeton's port in 1977 was the first [1].

And then, as well as Princeton's VM/370-based port and AT&T's TSS/370-based port for 5ESS development, Amdahl UTS came out in 1981 (descended from the Princeton port) and IBM/Interactive Systems VM/IX around 1984; then in 1988, the IBM/Locus AIX/370, followed by AIX/390.

(Also some 1980s-era Japanese supercomputers from Hitachi and Fujitsu had a hardware architecture based on IBM mainframes and some of them also ran Unix, although I'm not sure which version.)

> Anyway, as far as I can remember it was soon superseded by the Linux subsystem.

It was never superseded per se, it still exists. But the use cases are different. If you just want a vanilla Unix-like environment on an IBM mainframe, run Linux – which is not a z/OS subsystem, it is separate independent operating system. If you want a Unix application environment which supports deep integration into legacy mainframe applications and subsystems (VSAM, CICS, IMS, ISPF, etc) then that is the use case for the z/OS Unix subsystem.

In the latest release of z/OS, IBM has brought in a third option: zCX [2]. This runs Linux-based Docker containers on a hypervisor inside z/OS. This is positioned for z/OS system administrators to be able to easily deploy and manage Docker containers on the mainframe without learning extensive Linux skills (which managing z/Linux requires), using mainly their existing z/OS skills along with learning a bit about Docker.

[1] https://www.bell-labs.com/usr/dmr/www/portpap.pdf

[2] https://www.ibm.com/support/z-content-solutions/container-ex...