Though I have not done the type by type research, I think you would find a number of twins which are either not permitted, or not at home as high as 18000 feet. Though operation up there may be possible, it's really not a very nice place to be for very long. Even with supplemental oxygen, you'll get a headache, and otherwise just know that you're somewhere that your body does not want to be. I've crossed the North Atlantic four times in unpressurized aircraft between 15,000 and 18,000 feet, and after a few hours, you just want to be down again.
Yes, you might choose to hope over weather, or a mountain range, but even that is a rare need, and the aircraft manufacturers know it. If you're having to climb to 18,000 feet to clear weather, you're probably pushing your weather luck anyway.
During my certification testing work with Diamond, I remember having these discussions with respect to the DA-42. But, I don't really remember the content, as it was not relevant to what I was doing anyway. But many things are the aircraft manufacturer's choice, and it really simplifies their certification, and business in general, to just limit certain operations.