Isekai is literally the term for "other world".
Shield Hero is summoned, it's still an Isekai.
It's Portal Fantasy by another name.
looking at this, there has to be a complementing tag like reincarnation or transmigration, because some Isekai involves either. The case of Narnia is strictly a Portal Fantasy subgenre, whereas the Wizard of Oz is Transmigration trope.
Portal Fantasy mean the protagonist found a way to enter the other world, either through portal (duh) or through mysterious gate that's also a portal (duh, even). GATE did that, that one LN that blatantly says "I got a cheat skill in another world" something also does it. Hell, even Inuyasha counts as Portal Fantasy.
Other ways like Transmigration trope also quite popular, like Shield Hero or Arifureta, and who doesn't remember the good ol' Zero no Tsukaima?
The most popular trope for Isekai, however, is Reincarnation. Even among SH authors, it's the most popular trope within the genre.
sometimes it's being stuck in a VR game without being able to log out.
I call this 'barely an isekai'. Why? Because if it's based on SAO, the "Isekai" genre fell off halfway in Season 1. Now it's just Full Dive VRMMO trope with strong LitRPG element in it. Overlord, however, was a unique case.