I think the keyword here is 'unofficially'. If it were me, I would speak directly to the warranty provider (if it isn't the manufacturer) about whether a non-brand, aftermarket towbar could void your warranty. If you get hit from behind and the towbar causes damage, the insurer may also take the view that they won't cover damages as the towbar wasn't vehicle specific. It's fine and well telling you to save your money because they're expensive, but they certainly won't help you cover the potential future costs if something does go wrong.