Migrate abandoned/expired shortcode values to ACF field

traverse.php

<?php
/**
 * Plugin Name: Traverse
 * Description: A small plugin to look for specific abandoned shortcode in posts, get it's link (a shortcode attribute) and save that value in an ACF field.
 * Author Name: Junaid Ahmed
 */

add_action('init', function(){

	// post types to target
	$post_types = array( 'post' );

	$posts = new WP_Query(array(
		'post_type'		=> $post_types,
		'posts_per_page'	=> -1
	));

	while ( $posts->have_posts() ) {
		$posts->the_post();
		$content = get_the_content();

		// the following regex will match/get anything between the two strings i.e.
		// link=" and "
		// This regex is based on the shortcode below
		// [shortcode link="https://example.com/download/something.zip"]
		// so, this needs to change based on your expired shortcode structure
		$regex = '/link="(.*)"/';
		preg_match($regex, $content, $link);

		// $link[1] contains have our matched link i.e.
		// https://example.com/download/something.zip
		
		// check if there was a match
		if ( count($link) ) {

			// save the matched link to your ACF field
			// replace YOUR_ACF_FIELD_NAME with the ACF field you need to save the matched link in
			update_field('YOUR_ACF_FIELD_NAME', $link[1], get_the_ID());

			// while we are at it, why don't we remove the shortcode from the post
			// replace YOUR_SHORTCODE_NAME with your shortcode name and you're good to go
			wp_update_post(array(
				'ID'           => get_the_ID(),
				'post_content' => preg_replace('/\[YOUR_SHORTCODE_NAME(.*)\]/', '', get_the_content())
			));
		}
	}

	// assuming that our work is pretty much done
	// let's deactivate this plugin
	require_once( ABSPATH . 'wp-admin/includes/plugin.php' );
	deactivate_plugins( plugin_basename( __FILE__ ) );
});