I bet you’ve been using these your whole life without knowing it: in non-USA thermoses, the lid releases pressure before becoming mechanically free. You’d hear a “pop” and then the lid has to be twisted slightly further before it can be removed.
Not all safety features have to be obvious/obtrusive.
I see no hole in the metal anywhere on the inside. The only non-metal part that touches the inside is a rubber(?) gasket/ring thingy that sits above the thread and is squeezed by the lid, which I take out to clean and there's only metal under there as well. How would this work? Did I get the exceptional model that doesn't have one and just didn't look at other models closely enough?
Huh. I’d imagined the threading in the cap could be made this way