"Is FF14 an Isekai?" answered in order of expansion.

ARR - No, it is not an Isekai.

Heavensward - If Isekai is a metaphor for being somewhere that is not your home and feeling out of place, yes. Otherwise no.

Stormblood - Same as Heavensward.

Shadowbringers - Oh yeah. This is an Isekai.

Endwalker - FF14 is an Isekai and it has been since ARR.