A reader called my attention to this, a history and discussion of the tropes typical of the slew of recent anime, following on the heels of Sword Art Online, that depict a gamer otaku getting thrown into another world that looks like a sword-and-sorcery RPG.
This is a bit outside this blog’s usual scope; I admit my knowledge of this particular genre is minimal, simply because my interests run more toward shoujo anime, whereas isekai typically has male protagonists and a male target audience. However, the video does link isekai to predecessors from the ’90s like Magic Knight Rayearth and Vision of Escaflowne, which typically featured schoolgirls getting thrown into fantasy settings. Not discussed, but probably also an important influence on the isekai genre are the RPG-inspired fantasies from yesteryear like Record of Lodoss War.
In the last third of the video, the narrator explains that many of the light novels that make up the sources of isekai fantasy got their starts as web publications, and after suggesting that the isekai genre is saturated, he argues that this is leading in turn to a rise of “pure” fantasy without inserted modern characters, of which he holds up Made in Abyss as a premier example.
For recent examples of “traditional” or “pure” fantasy, I would also point to the less hyped but respectable Chaika the Coffin Princess, which was a competently made, light novel-based anime series that was something like an old-school fantasy in the tradition of Discarded Princess (because it was in fact from the same author and the same studio).
I have to confess I still haven’t watched or read Made in Abyss, though it’s on my list.