[dependenttypes]

It certainly is free. The standards are generally backwards compatible and the changes are simple. You do not even need to be aware about the differences between C89 and C11 to contribute to a C11 project.

> or because the toolchain they use elsewhere (personal projects, embedded comes to mind, or work) hasn't updated to the new standard.

gcc and clang both support it.

[cellularmitosis]

The latest gcc and clang might not be available on a particular platform.