The core of the AS/400 operating system, OS/400, the Licensed Internal Code (LIC), used to be written in another IBM-only PL/I dialect, PL/MP. In, the 1990s, while porting the AS/400 from CISC to RISC, they rewrote all the PL/MP code into C++. But IBM has another private dialect of PL/I called PL/MI, which was used to write the higher level part of OS/400, the XPF - basically PL/MI compiled to the virtual machine bytecode but PL/MP compiled to the machine code of the physical hardware. From what I understand, parts of the XPF are still written in PL/MP even in the latest versions of OS/400 (now called IBM i OS), although newer components tend to be developed in C/C++ instead. Other parts of the XPF were written in Modula 2, and I believe there is still some Modula 2 code surviving in the most recent versions as well.
IBM has had even more secret proprietary PL/I variants. The long defunct IBM 8100 line’s DPPX operating system was a written in PL/DS. Another dialect, PL.8, used to be used to write IBM’s compilers (although I believe it has now been replaced by C++) and also some of the firmware for the mainframes. IBM even wrote a GCC front-end for PL.8, although it appears they never released it.