Much like any topic, where the nature of the question is travel related, it's travel.SE.
So for example, talking about travelling to see auroras = travel. Asking about what causes them = science/astronomy - not travel.
However with travel and outdoors, there's always going to be some overlap, which makes it trickier. Many people go on holiday to go skiing, so they consider it travel and therefore ask it on here.
I don't see any problems with most of the ones you've cited on travel.SE, aside from the one that was migrated today. I notice that you actually linked to that one twice in your examples (both in the ones you think should be in outdoors, and the one you're not sure on).
Your [other one that looked dubious at first] about what to pack for a skiing trip - I first considered it very much outdoors, then noticed they weren't just asking about skiing equipment, they're asking about passport and ID stuff too, which makes it a bit more travel at least. However sadly none of the answers have addressed this, making it a bit more outdoorsy - including the accepted one. Others may have thoughts on this?