In 1983, AT&T released the fifth version of Unix, called "System V". Part of the release was an ABI specification for how the different parts of the system would talk to one another. Notably, the main portion of the spec described portable things like the file-format of executables, and the details for each supported platform were described in appendixes.

The SysV ABI is still used to this day, although the specification itself has withered until only two chapters remain[1], and CPU vendors still publish "System V ABI appendix" documents for platforms that System V's authors could not have dreamed of[2].

C as an interface is going to be around for a very long time, like POSIX and OpenGL and the SysV ABI standard. C as an actual language might not - it might wind up as a set of variable types that other languages can map into and out of, like what happened to the rest of the SysV ABI specification.

[1]: https://www.sco.com/developers/gabi/latest/contents.html

[2]: https://wiki.osdev.org/System_V_ABI#Documents