Also beyond the codebases themselves, interactions with the running code via GDB or tracing frameworks like dtrace, bpftrace, where you often reference C data structures. As well as being able to more fully understand the output of strace and other tools.
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Most people do that on Arduino which is as c++ as it gets, with templates, etc...
If you mean hobbyists, then probably yes. But there are whole industries who use embedded C as the defacto standard, such as automotive.
What can pay-off is learning to read C. Node.js, Firefox, Postgres, Linux are all written in C/C++ so if you ever want to really understand what's going on or have to debug a nasty bug, then you will need to be able to find your way around these code-bases.