IBM used Modula 2 to write parts of OS/400, the operating system of the IBM AS/400 – nowadays the operating system has been renamed to IBM i, (in part) since it now runs on identical POWER hardware to AIX. (At first, when they moved to RISC, they shared CPUs with the RS/6000 but had their own servers with incompatible firmware, only later did they unify the server lines and firmware so the same servers could run either operating system.)
In the original CISC version of the OS/400, the operating system kernel was mostly written in a PL/I dialect called PL/MP, but parts were also written in Modula 2. As part of the transition to RISC (PowerPC), large parts of kernel code were rewritten in C++ – although a significant quantity of PL/MP survived the rewrite, and a smaller quantity of Modula 2 – and apparently, to this day, some components of the IBM i operating system are still written in Modula 2.
My impression is, they wanted to replace PL/MP with a more mainstream language, and so started rewriting the PL/MP code in Modula 2. However, once it became clear that Modula 2 wasn't taking off, they switched to rewriting it in C++. But, typical "lava layer" anti-pattern, [0] all three of the original, the old rewrite and the new rewrite survive.
In the original CISC version of the OS/400, the operating system kernel was mostly written in a PL/I dialect called PL/MP, but parts were also written in Modula 2. As part of the transition to RISC (PowerPC), large parts of kernel code were rewritten in C++ – although a significant quantity of PL/MP survived the rewrite, and a smaller quantity of Modula 2 – and apparently, to this day, some components of the IBM i operating system are still written in Modula 2.
My impression is, they wanted to replace PL/MP with a more mainstream language, and so started rewriting the PL/MP code in Modula 2. However, once it became clear that Modula 2 wasn't taking off, they switched to rewriting it in C++. But, typical "lava layer" anti-pattern, [0] all three of the original, the old rewrite and the new rewrite survive.
[0] https://mikehadlow.blogspot.com/2014/12/the-lava-layer-anti-...