Email to notify disabled user

You can achieve this by adding a custom action hook to the disable_user_login.user_disabled hook exposed by the plugin.

That will provide you with the $user_id of the user that was disabled so that you can notify them via email if you wish.

Sample usage like so:

add_action( 'disable_user_login.user_disabled', function( $user_id ) {
       // Get the user
       $user = get_user_by( 'ID', $user_id );
       
       // TODO: Send the user an email telling them their account has been disabled.

       $subject = 'Your WordPress account has been disabled.';
       $message = 'Your WordPress account has been disabled.';

       wp_mail( $user->user_email, $subject, $message );
    }, 10, 1 );