Saying library authors are content with their absence is probably a bit too strong. There's not really another option, and I doubt many folks would choose not to author libraries for this reason alone. I personally have found their absence annoying when writing Rust libraries.
I'd probably be in favor of a hard cap on # of function arguments. :)
But there is another option: a configuration struct, potentially combined with an implementation of Default. Indeed, part of the reason this conversation has gone on so long is that there's one camp who just wants to emulate keyword arguments via additional sugar for structs and Default.
(There's also another another option, which is to have a different function for each combination of parameters. This obviously doesn't scale in the large, but it's perfectly acceptable for functions that take only a single optional parameter, which IME is a plurality of APIs that want optional parameters).
I'd probably be in favor of a hard cap on # of function arguments. :)