In my experience, it's only the cheap insurance policies bundled with credit cards and various memberships that consider high-altitude hiking a high-risk activity. Any travel insurance you buy as individual should cover it. If there is an altitude limit, it's usually 6000 m, so Everest Base Camp and Kilimanjaro would be covered but Aconcagua wouldn't. And any actual mountaineering would also be high-risk, regardless of altitude.