I wouldn't even call it a subgenre. It's a trope.

We do have that with owl house and amphibia, the Santa clause movies and series, and avatar, just sort of off the top of my head. None of these really have a sex harem (some of them have a single romantic interest tho), but nor did most of the anime I’ve watched, because anything involving a sex harem isn’t up my alley at all. Suuuuper cringe. But they do have the for-reasons overpowered self-insert main character in a fantasy land.

Arguably not enough to make a genre, ofc, but we do have some examples that fit better than the ones in the meme.

Edit to add - if anyone knows of more, I’m actually kinda into the genre, so I’m all eyes!

That’s just Japanese media, if it wasn’t catering to pedophiles then it wouldn’t get made over there

Part time weeblet here

Isekai if I remember right translates to other world. However in the context of the genre it’s when the protagonist gets transported to what is usually a fantasy world from what is usually death or reincarnation and sometimes summoning

Another trope I’ve seen is protagonist just wakes up in an MMO as their character and all the NPCs are sentient

I’m sure there are more but in anime it mostly boils down to: character is in fantasy now and knowledge of our world could give them an edge

When done right and done good it’s also a great trope for a fresh start and world building cus we don’t know the world, and the protagonist doesn’t know the world so we learn with them.

Not the 1865 Alice's Adventures in Wonderland which also has the same credentials but, as you can see, is earlier?

Still not sold on calling all of it Isekai regardless, but at least check your own facts

If you're going to apply that restriction you might as well go whole hog and also apply the "new world runs on shitty JRPG mechanics" part that also plagues modern isekai too.

I mean, Inuyasha exists.

Those are British though. Though I'm sure there are also American examples.