Zig supports this. If you compile with -fsingle-threaded, operations on mutexes turn into nops, atomics become simple loads/stores, etc.

Glibc has a bunch of tests throughout the codebase where it checks if there have been any threads started besides the main one. I don’t really know how effective they are from a performance perspective. (In principle, turning fgetc into getc_unlocked, for instance, could be quite beneficial.) Microsoft used to have a single-threaded C runtime, but it was done away with some time ago, I’m guessing because they started putting things into the platform that would start and manage random threads outside the programmer’s control.