Block WordPress Plugin Updates - best placed in wp-content/mu-plugins/

lock-plugin-updates.php

<?php
/*
Plugin Name: Lock Plugin Updates
Description: Allow blocking plugins from being updated.
Version: 1.0.0
Author: Example
Author URI: https://www.example.com/

@author John Doe
*/
    
add_filter( 'site_transient_update_plugins', 'prefix_lock_plugin_updates' );
function prefix_lock_plugin_updates( $value ) {
    if ( ! is_object( $value ) || ! isset( $value->response ) ) {
        return $value;
    }

    // format (relative to plugins dir): plugin/main-plugin-file.php
    $defaults = array();
    $locked_plugins = apply_filters( 'prefix_locked_plugins', $defaults );

    foreach( $locked_plugins as $plugin_basename ) {
       unset( $value->response[ $plugin_basename ] );
    }

    return $value;
}