code/controller/TimedNoticeAdmin.php
public function SearchForm()
    {
        $form = parent::SearchForm();
        $fields = $form->Fields();
        $q = $this->getRequest()->requestVar('q');

        $fields->removeByName('q[MessageType]');

        $fields->push(
            DropdownField::create(
                'q[MessageType]',
                'Message Type',
                ArrayLib::valuekey(Config::inst()->get(TimedNotice::class, 'message_types')),
                isset($q['MessageType']) ? $q['MessageType'] : []
            )->setEmptyString(' ')
        );

        $fields->push(
            DropdownField::create(
                'q[Status]',
                'Status',
                ArrayLib::valuekey(Config::inst()->get(TimedNotice::class, 'status_options')),
                isset($q['Status']) ? $q['Status'] : []
            )->setEmptyString(' ')
        );

        return $form;
    }
public function SearchForm()
    {
        $form = parent::SearchForm();
        $fields = $form->Fields();
        $q = $this->getRequest()->requestVar('q');

        $fields->removeByName('q[MessageType]');

        $fields->push(
            DropdownField::create(
                'q[MessageType]',
                'Message Type',
                ArrayLib::valuekey(Config::inst()->get(TimedNotice::class, 'message_types')),
                isset($q['MessageType']) ? $q['MessageType'] : []
            )->setEmptyString(' ')
        );

        $fields->push(
            DropdownField::create(
                'q[Status]',
                'Status',
                ArrayLib::valuekey(Config::inst()->get(TimedNotice::class, 'status_options')),
                isset($q['Status']) ? $q['Status'] : []
            )->setEmptyString(' ')
        );

        return $form;
    }
Update the SearchForm to use dropdown fields for MessageType/Status filters

@return Form