The ones composed on Earth went to Requiem to get turned into Knights. If the Didact ended up there, well, he'd actually be safe in his own shield world...
Yes. A lot of us are curious on what happened with the Didact as Requiem was cleared of Prometheans after Halo 4's campaign. And as evidenced by Episode 5 of SpOps, all the current Prometheans are humans from New Phoenix. Meaning that the Slipspace portal that the Composer was firing into sent the souls to Promethean Factories on Requiem.
