++++++++SPOILERS+++++++++++++++++++++ They didn't disappear, per say, they became apart of the dragon slayers, in order for them to develop anti bodies that prevented them from transforming into a dragon like Acnologia.
Well those weren't fused with the Dragon Slayers I believe some of them were slain سے طرف کی Acnologia and some left maybe are hiding from him یا something worse.
