Forgotten leave Tyria?
Hmmm, I’m not sure how you got that first bit of the Forgotten going to the Realm of Torment after Nightfall, as there was a sizeable population there for centuries before Nightfall (and during).
As we can best understand, the Forgotten population was at some point split into two – those who remained in the Crystal Desert (having all moved there in 174 AE) as Glint’s protectors, and those who lived in the Realm of Torment as Abaddon’s wardens.
Many, if not most or all, of the Forgotten in the Crystal Desert were killed by the player character and other heroes (Devona and co.) during Prophecies (3 years prior to Nightfall). We held absolutely no hint or indication at their population numbers afterwards – or even if they resurrected themselves afterward (possible but unproven). All we know is that there’s been no mention of them since, not even during Edge of Destiny at Glint’s lair.
After Abaddon’s fall, those in the Realm of Torment claimed that their duty has ended and they were free to go elsewhere, though we know some remained behind to take out Mallyx. Like those of the desert, we hold no indication of what they did after Mallyx’s defeat.
Stop treating GW2 as a single story. Each Season and expansion should be their own story.