Let's play a game: Save the Ancients!

[Iscah]

The events of Shadowbringers resulted in an entirely new timeline, not a mere branch. There is no conjunction formed for the 8UC with Elpis, and so Elpis is not tied to that timeline in the same way as it is in ours.

The reason it's not a mere branch is because the time at which the conjunction occurs is hundreds of years before the timeline split happens. The timeline split happens because the 8UC timeline sent the Crystal Tower backwards through both time and space, causing an entirely new timeline to form.

There is absolutely nothing to establish that as fact. There is no proof that the Crystal Tower's arrival split the timeline, because we have no prior knowledge of its state to say whether there was ever a version of events where the tower did not appear there.

Neither is there any proof that the split is an entirely separate timeline and not just a split.

And without any direct proof for or against, I would take "the interpretation that allows the game's story to make sense" as the one that is likely to be in place rather than the one that renders parts of it impossible or inconsistent.