Required field only if another field has a value, must be empty otherwise

You have to create your own validation rule.

Edit app/Providers/AppServiceProvider.php and add this validation rule to the boot method:

// Extends the validator
\Validator::extendImplicit(
    'empty_if',
    function ($attribute, $value, $parameters, $validator) {
        $data = request()->input($parameters[0]);
        $parameters_values = array_slice($parameters, 1);
        foreach ($parameters_values as $parameter_value) {
            if ($data == $parameter_value && !empty($value)) {
                return false;
            }
        }
        return true;
    });

// (optional) Display error replacement
\Validator::replacer(
    'empty_if',
    function ($message, $attribute, $rule, $parameters) {
        return str_replace(
            [':other', ':value'], 
            [$parameters[0], request()->input($parameters[0])], 
            $message
        );
    });

(optional) Create a message for error in resources/lang/en/validation.php:

'empty_if' => 'The :attribute field must be empty when :other is :value.',

Then use this rule in a controller (with require_if to respect both rules of the original post):

$attributes = request()->validate([
    'a' => 'required',
    'b' => 'required_if:a,x|empty_if:a,y,z'
]);

It works!

Side note: I will maybe create a package for this need with empty_if and empty_unless and post the link here

Required if

required_if:anotherfield,value,... The field under validation must be present and not empty if the anotherfield field is equal to any value.

'b' => 'required_if:a,x'