The Isekai anime has a bigger history than most people realise but did you know that one of them is Mario.

Back in 1986 while Mario was still finding it's audience with the fledgling Famicom system, an OVA was produced entitled, the Mission to Save Princess Peach, Mario isn't even a plumber, instead being a convenience store worker; the OVA really gets going when the Famicom he's playing spits out Peach and Bowser, after a skirmish, Peach leaves behind a pendant so the Mario Bros following a canine companion enter the Mushroom Kingdom through the TV.
This predates many well known Isekai, even ones like Rayearth, Digimon and Monster Rancher who are not normally associated with the genre but their plots suggests otherwise.
Mario's anime plot plays into the western cartoon series as the Super Mario Super Show has the plumbers originate from Brooklyn and enter through the warp pipe into the Mushroom Kingdom, in fact Nintendo built much of it's identity on Isekai as the end of the 80s brought in Captain N the Game Master where a teenager enters the video game world packed full of Nintendo properties like Zelda, Metroid and Kid Icarus.
The funny thing about all this is that Nintendo never really stopped with the Isekai, re-emerging in the Pokemon Legends Arceus late last year.
It's interesting to see how frequently this well known plot turns up.