It's optional for the caller, but not for the callee. I think the controversy centers around two things:
a) Specific to this feature: For library maintainers, this means that argument names are now part of the public API, whether I like it or not, and renaming a parameter from $orderBy to $sortBy is now technically a breaking change. Yeah, you can put something in the readme saying "Heads up: I don't support named arguments and intend to rename arguments in semver-minor releases", but should you need to do that?
b) General to all new PHP features: PHP is a great language, with lots of crap code written in it. Will this feature result in people writing better or worse code? Just because something is optional doesn't mean it's a good idea, if it will result in the overall code quality of the ecosystem to get worse. Some people think that named arguments will make it too easy to create monster functions with too many parameters.
I personally support (aka am DELIGHTED by) named arguments, but I'm sympathetic to the concerns of library maintainers and I don't see a better way around it than a remark in the readme (at least until/unless they introduce e.g. a @@NoNamedArguments annotation in PHP 8.1). And I think that the benefits of named arguments far surpass the risk that they encourage people to create methods with too many arguments. But I can see both arguments.