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.

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

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