I don't know anything about Zig, but I believe that a good reason for cURL to be written in ANSI C is that even your toaster ships with a C compiler able to build the source code.
If that is in fact the reason for sticking with ANSI C then I would like to advocate for a language that compiles to C. In which case I would like to suggest Nim[1].
Of course, I don't think that curl should be rewritten in Nim. But since we are advocating for other languages I thought I would pitch in. If anything, writing new curl functionality in Nim would be the way to go. That is assuming that curl's author is unhappy with C, which does not seem to be the case.
If you depend on Zig then you only depend on Zig to produce a binary or library for any system.
If you depend on Nim then you depend on Nim, a C compiler, and a linker. And I hope your C compiler supports cross compiling out of the box. It probably doesn't.
Of course, I don't think that curl should be rewritten in Nim. But since we are advocating for other languages I thought I would pitch in. If anything, writing new curl functionality in Nim would be the way to go. That is assuming that curl's author is unhappy with C, which does not seem to be the case.
1: https://nim-lang.org