M1: Import Values to Dropdown/Multiple Select Attributes - https://keithgreer.dev/import-values-to-dropdownmultiple-select-attributes

import-values.php

<?php
require_once '../app/Mage.php';
umask(); Mage::app('default');

$_manufacturers = file('import.txt');
$_attribute = Mage::getModel('eav/entity_attribute')->loadByCode('catalog_product','manufacturer');
$manufacturers = array('value'=> array(),'order'=> array(),'delete'=> array());

$i = 0;

foreach($_manufacturers as $_manufacturer){
  $i++;
  $manufacturers['value']['option_'. $i] = array($_manufacturer);
}

$_attribute->setOption($manufacturers);

try{
  $_attribute->save();
  echo 'Attribute values successfully imported';
} catch(Exception $e){
  echo 'Import Error::'.$e->getMessage();
}