There's a lot of mention of Sword Art Online being the anime that made Isekai but back in the 90s it was a mostly Shojo Genre.
How far back does it go?
Rumiko Takahashi in 1985 released a short anime called Fire Tripper in which a girl is gas exploded back to feudal Japan, alot of elements were later reused for InuYasha, more on that later. Any anime adaption of Alice in Wonderland and Wizard of Oz counts as well.
What are the Main Anime?
Four big names of yester year spring to mind, Vision of Escaflowne, Fushigi Yuugi the Mysterious Play, Magic Knight Rayearth and Inuyasha, all four feature a female heroine drawn into another world either through dimensions, a magical book, summoned by a preist or dragged backwards through time respectively.
How does Digimon fit into the equation?
Digimon by definition is also an Isekai by the fact that a group of 7-8 kids get sent to the Digital World but has more roots in the monster trend alongside another well known title, Monster Rancher.
Staying on topic how good are these four female led Isekai?
Escaflowne has argubly the most beautiful score ever composed for an anime.
Fushigi Yuugi was instrumental in establishing the Josei genre.
Magic Knight Rayearth put the artist team of CLAMP on the map as one of their best work.
InuYasha is yet another stellar work by Rumiko Takahashi and her best selling series by far.
What went wrong?
Escaflowne had a botched dub release and is widely forgotten next to stable mates Cowboy Bebop and Outlaw Star, not to mention the plot made no sense and the manga is infinitely different.
The others out stayed their welcome, Rayearth had a dreadful 2nd season, Fushigi Yuugi has not aged well and has been out of the spotlight for two decades now, InuYasha kept going and going and became the punching bag in the awkward era of the 00s where anime was struggling with a hangover, it's also aged the worst of the four shows.
As for Digimon, each season was getting less and less returns until they decided on Digimon Tri and a season 1 reboot.
Did Sword Art Online save Isekai?
Kind of, it's premise added elements that weren't available in 90s Isekai and took inspiration from harem, shonen and even thriller in some cases, Sword Art Online also took inspiration from Dot Hack Sign.
Now we can't get rid of Isekai
Blame covid, Japan's abysmal birth rate and Sword Art Online outstaying it's welcome, on the bright side, female led Isekai made a return with Saints Magic is Omnipotent and to a lesser extent Bofuri, we also have enough try hards to keep anime interesting and it's not devoid of having it's formula mixed in ever more dangerous ways, it's not like the tail end of the early 2000s where anime actually stagnated.
