The hygiene hypothesis is still unproven and controversial
That's not quite correct; "unproven" is a confusing word here. It's more of an "it depends" situation, rather than a "true/false" situation.
The hygiene hypothesis can be sort-of demonstrated in some situations (e.g. reduced allergic response to peanuts in mice via oral sensitisation with very low amounts of CpG-coated peanut extract), and rejected in others (e.g. the parasitic worm H. polygyrus suppresses the adaptive immune response).