The UNIVAC 1108 (and descendants) mainframe architecture was not discontinued in 1986. The company that owned it (Sperry) merged with Burroughs in that year to form Unisys. The platform still exists, but now runs as a software emulator under x86-64. The OS is still maintained and had a new release just last year. Around the time of the merger the old school name “UNIVAC” was retired in a rebranding, but the platform survived.

Its OS, OS 2200, does have a C compiler. Not sure if there ever was a C++ compiler, if there once was it is no longer around. But that C compiler is not being kept up to date with the latest standards, it only officially supports C89/C90 - this is a deeply legacy system, most application software is written in COBOL and the OS itself itself is mainly written in assembler and a proprietary Pascal-like language called “PLUS”. They might add some features from newer standards if particularly valuable, but formal compliance with C99/C11/C17/C23/etc is not a goal.

The OS does contain components written in C++, most notably the HotSpot JVM. However, from what I understand, the JVM actually runs in x86-64 Linux processes on the host system, outside of the emulated mainframe environment, but the mainframe emulator is integrated with those Linux processes so they can access mainframe files/data/apps.