How do I add nopin=”nopin” to the default avatar?

You can try this one:

function wp_ste_add_nopin_args( $args ) {
    $extra_attr="nopin="nopin"";
    $args['extra_attr'] = $extra_attr;
    return $args;
}
add_filter( 'get_avatar_data', 'wp_ste_add_nopin_args', 999, 1 );

For further reading, please visit: https://developer.wordpress.org/reference/functions/get_avatar_data/