add_filter(
   'notify_user_about_new_follower',
    function(
        bool $true,
        $user,
        $actor,
        $activitypub_object,
        $follower
    ) {
        // Your code here.
        return $true;
    },
    10,
    5
);

apply_filters( 'notify_user_about_new_follower', true, $user, $actor, $activitypub_object, $follower )