I've never seen a thermos bottle with a relief valve. Not sure if all manufacturers in Europe have nonexistent QA or if it's just not a thing that's commonly needed since you can't heat the contents up in there anyway (it's not like an insulated water boiler, which does have a release valve). Never considered that someone might leave food in there long enough to build up explosive pressure. Feels a bit like saying a missing cat detector function in microwave ovens points at them having no quality control