add_filter(
    'mastodon_api_notifications_get',
    function (
        array $array,
        $request,
        $limit,
        $before_date,
        $after_date
    ) {
        // Your code here
        return $array;
    },
    10,
    5
);

apply_filters( 'mastodon_api_notifications_get', array(), $request, $limit, $before_date, $after_date )