I'm finally doing the questline to get the Haurchefant emote where you go around to various places, essentially reliving the story. Well there's a part where you go see Tiamat, and after Endwalker Tiamat isn't there. Clicking on the location node still plays a cutscene with Tiamat in it, but I figured these kinds of things happen, no big deal. Except the last line of the cutscene is "..Perhaps there was hope for them. Perhaps, this time, they would learn and remember. But her part in the tale had ended long ago. Within this monument to hubris would she remain until the end of days...beyond redemption..."
And then I came out of the cutscene and Tiamat was gone. I don't know if it's worth it for the devs to make an alternate thing for post EW players, but I thought it was worth mentioning at least.