Code to Disable All Types of Automatic Updates in WordPress

/* Completely Disable Automatic Updates in WordPress */
add_filter( 'automatic_updater_disabled', '__return_true' );

To completely disable all types of updates in WordPress (sitewide) including core, theme and plugin updates, add this code to your child theme’s functions.php. OR, you can also use the below code in your wp-config.php instead of editing the theme functions file. Even if you change themes, auto updates will always be disabled.

/* Disable All Types of Updates */
define( 'AUTOMATIC_UPDATER_DISABLED', true );

This code also removes the Enable Auto Updates option in WordPress Plugin List page.

Note: Once this code is added, each and every update needs to be done manually.