add_filter(
   'notify_user_about_lost_follower',
    function(
        bool $true,
        $user,
        $actor,
        $follower
    ) {
        // Your code here.
        return $true;
    },
    10,
    4
);

apply_filters( 'notify_user_about_lost_follower', true, $user, $actor, $follower )