Ascians can lose their immortality if their souls are destroyed by being absorbed into auracite and then hit with an intense blast of aether. Emet-Selch is killed at the end of the Shadowbringers main scenario when shards of auracite stab his body and the player character hits him with the Light that had accumulated in their body, dissolving into aether. Despite this, Hydaelyn is still able to bring his soul back to the First along with Minfilia’s, and the player character is able to summon him before the final battle of Endwalker. At the end of the healer role quests in Endwalker, Fordola experiences a vision of Charlet passing on to the afterlife along with her deceased friends, despite the fact that he had been turned into a blasphemy, which we know involves being drained of all (or almost all) of their aether. This occurs even if you complete the role quests before defeating Meteion.

This leads me to believe that while someone’s soul can be destroyed if its aether is expended or scattered, the aether itself still remains in existence and is able to rejoin the Lifestream, where it can eventually reform. This is supported by Montichaigne’s theory that souls within the aetherial sea are reduced to pure aether and combine with aether from other souls to create new souls, which he considers just as likely as souls remaining whole before reincarnating. We also know that souls that remain in the Lifestream for long enough end up dissolving into the greater flow of aether. The fact that Emet-Selch refers to his and Hythlodaeus’ souls as “half-faded” when they’re summoned by the player may imply that this has already started happening to them.
When Hydaelyn dies, we see her remaining aether crumble apart and disperse into the aetherial sea, meaning Venat may very well have been able to return to the star despite her soul no longer being whole. From a narrative perspective it would make more sense, considering that when the player meets her in the past she refrains from returning to the star because she feels that she still has work to do and when she sees that the player is capable of stopping Meteion in the present she finally feels at peace with dying. Elidibus dies in a similar manner, using the Crystal Tower to transport the player back in time despite knowing that his soul will be consumed in the process. Despite this, he seems to believe that he will reunite with his friends, which he wouldn’t think if the destruction of his soul meant the destruction of its aether and he could never return to the Lifestream. At the end of the credits of Endwalker, Venat can be seen walking alongside Emet-Selch, Hythlodaeus, and Hermes, looking over at the player character as the Scions pass by and smiling in contentment. While this scene can be interpreted as a symbolic passing of the torch, I believe that it could also signify her being in the same place as the other Ancients, finally having finished her duty and returned to the star.