yea i figured there was an older one. Inuyasha is just the one i can always remember off the top of my head. Hades there's probably one even older than Rayearth. Maybe one of the Clamp series.
You really want me to put here all the '' Isekai '' manga/anime created since the last ten years ( we can reduce that to five actually ) since Inuyasha ? ( Please don't ! )
It's a trend , not a genre.
nah its still a genre. Isekai is the newest genre that gets a decent amount of series for it. before isekai it was Moe series or it was magic/supernatural academy series or it was slice of life series or it was mecha series or it was etc. etc. no matter how much you deny it, isekai is a genre just like mecha is or like comedy or like supernatural or like the many other genres and subgenres. also while isekai is a decent sized genre, in terms of series amount, it's still dwarfed in the shear number of anime that come out every season. Hades in the upcoming season there will be about 6-7 new isekai series and thats out of 50-55 new series. about half of those will be new seasons of an isekai thats had a previous season in a different anime season.
as for how many have come out in last 5 years probably less than 200 anime series/seasons have been an isekai and thats out of at least 1000 anime series/seasons that have come out in that time period. face it, isekai is just one of the many genres out there and itll probably stop being the genre that gets as many as it is when the newest genre gets a boost in popularity and then you can start whining about that one like you probably whined about moe or magical girl or magic/supernatural academy genre being popular.