Quote Originally Posted by Iscah View Post
It wasn't even from within the game, just from early promotional stuff for Shadowbringers.

There was never any hint in the game, so far as I recall, that either the Ascians or the Scions saw any significance in the eighth Rejoining as a turning point in the process. Again, that's just what fans speculated might be the reason why the devs hinted that it would be the last.

As it turned out, that "eighth and final" calamity was the particularly catastrophic Black Rose event, which (at least at the time they said it) they may have intended to mean either the last calamity in that timeline before it was undone, or possibly even the last one before humanity was wiped out entirely by the lingering effects of it.
In the Heavensward post patches, you speak with Minifilia/Hydaelyn who has now become the Word of the Mother, and here she explains that she has failed to stop the 7 previous calamities/rejoinings and you can't let them do it once more because she won't have the energy. So the basic idea is in place pretty early in the story, granted not as fleshed out as we come to understand in Shadowbringers.