What about aggregate literals, designated initializer syntax, _Generic, and thread.h?

Variable-argument macros, function prototypes (well, modules would be better…), // comments, stdatomic instead of 'volatile' hacks, _Bool, aligned_alloc, …

I’d add the deprecation of register. Not an innovation but an improvement nonetheless.

Interestingly, the high level language you implement for the nand2tetris course has this constraint, which makes writing the compiler easier

I took that course, too! I loved it. Learned more about computers than any other of the many courses I took.