Hi Product Hunt, I made Isoline.

A lot of altitude sickness comes from climbing too fast, not from being unfit. Many travelers fly from Lima, at sea level, to Cusco at about 3,400 m in just over an hour, spend the first afternoon walking around, and wake up the next morning with a splitting headache. Once you're up there and feeling off, it's hard to tell whether to keep going, rest, or go down.

What Isoline does:

Before you go: look up the altitude of any place, or enter a driving route to see the towns and passes along the way, with suggested places to sleep so you don't climb too fast. Built-in routes include the Everest Base Camp trek and Kilimanjaro's Machame route.
At altitude: it notices when you pass 2,500 m. The check is four symptom questions from the Lake Louise score, four warning signs, and a recent blood oxygen reading from your Apple Watch. You get one answer: keep going, rest, or descend now, with the next check-in reminder already set.
If it's serious: clear steps to head down, a button to call the local emergency number, and a text with your location and altitude to your emergency contact.

There's no account, and your health data never leaves your iPhone and Apple Watch. It's free.

Some limits, up front: the check needs an Apple Watch that measures blood oxygen, route planning only works for driving routes for now, and it's not a medical device.

If you've been to high altitude, I'd like to hear what you wish you'd known before going.