Após postar minha pergunta, descobri que (embora não seja um rótulo de gênero em si) a Lost Colony
A human colony on another planet experiences a disaster which destroys its tech-base, and for some reason the rest of humanity never checks up on that colony with which it suddenly lost all contact. Alternatively, a spaceship having nothing to do with colonization experiences an emergency which forces its crew to land on a planet in an uncharted star system, and for some reason they never get rescued.
Either way, not only do their descendants' politics, economics and culture regress to match their pre-Industrial Revolution technology level, they also forget that their ancestors ever came from another planet, making the story at first glance seem to be set in a pure fantasy world. Oftentimes whatever remains of the old technology will be mistaken for magic by the colonists' descendants. Sometimes there is genuine supernatural magic happening, or something that can pass for it, and all advanced technology has been lost. And sometimes the old technology and genuine magic are used side by side.
Os exemplos na TV Tropes incluem Bitterblooms , Might & Magia , e Warhammer , então eu acho que essa é uma partida bem próxima. Se não conseguirmos encontrar outro rótulo de aproximadamente igual ou maior especificidade, o nome do tropo é uma resposta boa o suficiente para mim.