How do you know nobody uses them? I'd be willing to bet that there are many older in-house scripts puttering away in some dark, webbed corners of internal networks, which rely on this functionality, e.g. because they are getting their input from some obscure network appliances which print IP addresses like that.

I would fully expect support for these "weird" formats to be a (enabled by default) build-time option, but it's a good thing that curl maintainer is not just cutting it off right away.