By default Mailster will remove all unused tags in the final version of your campaign. To prevent that you can whitelist certain tags with some custom code.

add_filter( 'mailster_keep_tags', function( $tags ){
    $tags[] = 'my-special-tag';
    $tags[] = 'my-other-tag';
    return $tags;
});

This will keep {my-special-tag} and {my-other-tag} in you campaign. It will still remove variations like {my-special-tag:123}.

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