By the 1970s, IBM was rewriting its flagship operating system in PL/S. Although users frequently asked IBM to release PL/S for their use, IBM refused saying that the product was proprietary. Their concern was that open PL/S would give competitors, Amdahl, Itel (National Advanced Systems), Storage Technology Corporation, Trilogy Systems, Magnuson Computer Systems, Fujitsu, Hitachi, and other PCM vendors a marketing advantage. However, even though they refused to make available a compiler, they shipped the PL/S source code to large parts of the OS on microfiche to customers, many of whom thus became familiar with reading it.
Closed PL/S meant that only IBM could modify and enhance the operating system.
In the mid-1970s, a single programmer, working from publicly available documentation, wrote a fully functional PL/S compiler. IBM legally suppressed this software.
This was RAND corporation. They made the mistake of using internal IBM documentation for their development, which they did not have permission to use for this purpose, and IBM threatened to sue them, so they never offered the product for sale. They did however advertise its imminent release through SHARE meetings.
PL/S was succeeded by PL/AS (Programming Language/Advanced Systems), and then PL/X (Programming Language/Cross Systems). PL/DS was a closely related language used to develop the DPPX operating system, and PL/DS II was a port of the S/370 architecture for the DPPX/370 port.
As the market for computers and software shifted away from IBM mainframes and MVS, IBM recanted and has offered the current versions of PL/S to selected customers (ISVs through the Developer Partner program.)