This, I also feel like pop culture has forgotten overtime just how much inspiration (and in many cases ripping it out of the source material 1:1) early D&D took from science fiction / fantasy mashup classics like Jack Vance's Dying Earth.
There's not much difference to a common denizen of a fantasy world between a wand that shoots bolts of lightning and polished chrome laser pistol tube, or between a flying carpet and hover car. It's all the same to them, and the whole "our ancestral civilizations were so knowledgeable in the lost occult arts that they could shape magic into common objects" when it's really just forgotten high tech sciences is always a fun bit of world building.
This kind of material absolutely belongs in fantasy and I really hope that someday sci-fantasy makes a big comeback, some of my favorite reading books and inspiration for tabletop material have come out of them.
Dying Earth and the Book of the New Sun would be the big two I'd recommend, though their settings are very similar they're very different in tone. Book of the New Sun would be better if you want a more serious read about that kind of world while Dying Earth is more of a collection of (very) humorous and whimsical short stories about a bizarre setting and it's even more bizarre inhabitants.
Hard to be a God was a good read I enjoyed and feels like it falls into this category as well, from the point of the "higher beings."
A lot of famous early pulp fantasy didn't make as much of a concrete distinction between science fiction and fantasy either and tended to blend the two freely. I think most people would think of Conan as pure fantasy but there were a handful of stories about him encountering otherworldly alien beings and sci-fi technology, or ancient highly advanced civilizations whose progress is incomprehensible to (in Conan's world) modern inhabitants. IIRC there was at least one Fahfrd and the Grey Mouser story where they come across an interdimensional traveler.
If other people have recommendations too I'd love for them to chime in. It's a genre I feel is definitely underrepresented.
The Fafhrd & Mouser story (one of them, anyway) is The Swords of Lankhmar, which is absolutely fantastic.
Michael Moorcock is another writer who often mixed fantasy and SF. His Dancers at the End of Time series is brilliant and hilarious. Or more straight-faced and pulpy, the Hawkwind and Elric stories are great.
And Jack Vance, of course. As well as The Dying Earth, he did a bunch of comedy-oriented stories in the same setting, starring Cugel the Clever and Rhialto the Marvellous.
There's not much difference to a common denizen of a fantasy world between a wand that shoots bolts of lightning and polished chrome laser pistol tube, or between a flying carpet and hover car. It's all the same to them, and the whole "our ancestral civilizations were so knowledgeable in the lost occult arts that they could shape magic into common objects" when it's really just forgotten high tech sciences is always a fun bit of world building.
This kind of material absolutely belongs in fantasy and I really hope that someday sci-fantasy makes a big comeback, some of my favorite reading books and inspiration for tabletop material have come out of them.