Context:

The Oxford English Dictionary has added 23 Japanese words, including "isekai"
The Guardian: The Oxford English Dictionary’s latest update adds 23 Japanese words

IGN wrote an article where they suggest that there is a shift in the manga/anime community where the meaning of "isekai" is changing to any story set in another world and that stories where a character is transported to a different world should be called "isekaitensei".
Frieren would thus be considered an isekai.
Twitter: Isekai, one of the most popular genres in anime and manga, is now officially in the Oxford English Dictionary. However, it may not mean what you think. https://bit.ly/49k7jZY
IGN: Isekai, the Popular Manga and Anime Genre, Is Now in the Oxford English Dictionary

Their source for this article is a Twitter conversation by Celina.
https://twitter.com/tkn0801/status/1768097670470697403

GabrielWB said:

Context:

IGN wrote an article where they suggest that there is a shift in the manga/anime community where the meaning of "isekai" is changing to any story set in another world

English already has a word for that: FANTASY.