Any established C codebase, for example the kernel or Postgres?

Traditionally microcontroller firmwares as well, though those are increasingly friendly to C++, you just have to be careful about allocations as C++ makes it way easier to accidentally allocate than C does.

literally a good majority of existing embedded software coupled to applications in safety -- devices used by fire safety and first responders.

Writing extensions for projects that support C extensions but may not support C++ extensions, e.g. many dynamic languages.

Embedded systems, for example.