感谢ramy herria，我能够将他的答案扩展到formrequest类：

protected function validationData()
{
    $all = parent::validationData();
    //Convert request value to lowercase
    $all['choice'] = strtolower($all['choice']);
    return $all;
}

public function rules()
{
    $choices = $this->route('modelName')->choices;
    return [
        'choice' => [
            'required',
            //Also convert array to lowercase
            Rule::notIn(array_map('strtolower', $choices))
        ]
    ];
}

您可以将输入数据和当前数据小写如下：

$input = request()->all();
$input['choice'] = array_map("strtolower", $input['choice']);
$this->validate($input, [
    'choice' => [
        'required',
        Rule::notIn(array_map("strtolower", $choices))
    ]
]);

您可以编写自己的验证规则类：

use Illuminate\Contracts\Validation\Rule;
use Illuminate\Validation\Concerns\ValidatesAttributes;
use Illuminate\Validation\Rules\In;

class CaseInsensitiveInRule extends In implements Rule
{
    use ValidatesAttributes;
    private const FORMAT_FUNCTION = 'strtoupper';

    public function __construct(array $values)
    {
        $this->values = array_map(self::FORMAT_FUNCTION, $values);
    }

    public function passes($attribute, $value)
    {
        $value = call_user_func(self::FORMAT_FUNCTION, $value);

        return $this->validateIn($attribute, $value, $this->values);
    }

    public function message()
    {
        return __('validation.invalid_value');
    }
}

接下来可以在请求类中创建一个对象

public function rules(): array
{
    return [
        'status' => new CaseInsensitiveInRule(['active', 'deleted'])
    ];
}