Isekai just means you have a char from the real world being transported to some other world. That's what the term means; "other/different world". It doesn't need to be orthodox fantasy or a dragon quest parody with slimes and the demon overlord to be isekai, and the protag doesn't need to be OP or have a harem either. In fact, Re:Zero, one of the best isekai, has a pathetic protagonist who dies all the time, and his power is just reviving a bit before dying at a "checkpoint" and remembering how he died, and trying to not die this time around, over and over until he manages to reach a new checkpoint.
Some older stuff which is Isekai but people didn't term it as such:
Alice in Wonderland
Magical Knight Rayearth
Inuyasha
Final Fantasy Tactics Advance
Digimon
Sen to Chihiro no kamikakushi/Spirited Away