Most travel insurance that covers 'hiking' would be fine.
Read the fine print.
All depends on whether your actually hiking or 'mountaineering' / 'climbing' etc.. If your just carrying a pack on your back and not using ropes or doing anything particularly out there, altitude is okay to a point. then generally standard insurance would be fine and cover rescue costs, provided you follow the rules of the policy.
e.g. they may insist on certain conditions are followed in an emergency situation.
I've used Travel Insurance Direct several times for standard stuff, e.g. backpacking and hiking in South America.
If I'm dropping backcountry lines Snowboarding in Europe or the US - I use BUPA IHI
https://www.ihi.com/They offer coverage for a range of more extreme stuff and better rescue coverage. I go for this one if I think the chances of ending up in a spot of bother are higher than normal.
When it's all said and done, I don't trust insurance companies.. they've weaseled their way out of a few travel insurance claims I've made in the last 15 or so years.
Stuff that was just annoying and too much trouble to argue or pursue. So don't automatically assume your always covered.
i.e. the motor cycle one always catches the bogans out in SE Asia.
Also the effort required to claim sometimes is quite a great deal of work.. e.g. police reports, translations of police reports, bank statements.. etc.. oh, btw. you have to wait a week to get the police report and you need to do this in person because it needs to be officially approved.. then the details of the report are wrong because of language / translation issues. Having to explain to insurance company. Then needing to follow due process to get the claim recognised..
Its not necessarily a case of I have insurance therefore I'm covered. But generally I think they're pretty good as far as medical coverage goes.