[ChoGGi]

"you would have the most awkward ugly installation that blocks your upper cabinet doors"

No, you route it through the cabinet and if you don't want it to look ugly then you build a chase around it. I usually don't since the chase takes up more room, and no one really cares as it's in the cabinet.

And yes they are designed for venting outside, that's what these are for: https://www.homedepot.com/p/Master-Flow-3-1-4-in-x-10-in-x-7...

[Spivak]

We’re talking about two different kinds of microwaves. There are the ones you describe where the vent is inside the upper cabinet, those are designed to be vented outside. Then there’s the microwaves where the vent is in front of the cabinet doors, runs along the whole length of the microwave and blows up at the outside of the upper cabinet doors. There is nowhere you could even route it because anything on top of the microwave to capture it would physically block the doors from opening.