They are written in C unless they are powerfull enough to hold something else, like Cisco does with Chicken Scheme on their routers.

https://www.micrium.com/rtos/tcpip/

Quoting the relevant quote "you wouldn't use C to write your simple website", except that is exacly what EE does when on device memory is measured in KB.

C++ would be safer, but the C89 culture reigns in such domains.