Yes, in various ways, but for application code it’s generally not the bulk of it though there may still be individual C source files in there. OpenGL shaders, OpenCL and Metal Shaders are basically written in C as well. It’s technically a separate language but it’s based on, I believe C99. Though remember that while it doesn’t change dramatically, C and C++ are still alive languages and do get evolved, even if it’s not like a full modernization at least for standard C.
But yeah you might fairly often find an algorithm in C that is called from another language. Swift is very good about C COmputability as well so you can fairly easily call a C function and such. There are also a lot of open source projects out there that you can use as dependencies that will have C codee
It’s a good language to have some familiarity with though not necessarily expertise. I doubt there’s much in user land written solely in C, but C interop is prevalent.