I find it useful in lower powered systems without floating point hardware. It may be a feature whose time has passed but it still seems like there's both a case for it and probably an elegant design waiting.

Imagine using it on a 6809 system for example. :)

You could still do it as a userland feature and implement add/sub/mult/div as methods on the type.

That never works as well as having it built into the compiler.

You can define your own "built-ins" types and functions(!) so that should make it indistinguishable.

You can also extend the standard Library from inside your own project, modules are open for extension

Well, you're free to revive this issue: https://github.com/c3lang/c3c/issues/1451. I'd need a more solid proposal.