Western Planetary Romance (Aka Isekai before the Japanese term became popular) were more often a sci-fi setting than fantasy. Buck Rogers in the 24th century f'rex, where Rogers was put into suspended animation by a mysterious gas in a cave in and woke up five centuries later. Sherlock Holmes in the 22nd century is a bit of a pastiche where Holmes' body is enbalmed in honey and future science is able to revive him, with his sidekick an android that's memorized everything Watson ever said and programmed itself to have the same attitude. Gordon R. Dickson's short Story "Danger, Human" has the MC abducted by aliens, as do the execrable Gor novels. Actually a lot of the sci-fi Isekai start with alien abductions.
One thing we're running into is that with the breakdown of traditional publishing houses to act as gatekeepers, anybody can publish their own webnovel and get enough recognition to be published. I don't think this is necessarily a bad thing, mind. If you read and clicked like on "In another world with my cheat power" all the search engines will promptly feed you "I died but now my cheat power got me a harem" and "I went to another world and now I'm the demon lord?" These two factors together have caused literature to become more "faddish." The search engine boost means whenever a really popular original story comes along, all the inevitable copycats get boosted by the search algorithm applying the same tag to them. The lack of publisher control means that everybody who got boosted can get published. This tends to lead to the same fad idea suddenly blossoming all across the world at once and then dying out once people get sick of it and a new idea blossoms everywhere at once.
The same effect also tends to magnify stories with the exact same premise because the search engines tend to run off of tags, so a story that has 100% of the tags of the very popular "In another world with my cheat power" will get to the top of the similar stories list where as "Aliens Kidnapped Me and Now I'm a Mercenary Commander" will only share maybe a quarter of the tags, not get the same popularity boost, and be lost down in page 4.