The plot did not forget he existed...the same way Kishi did not forget Sasuke when he was missing for all those chapters due to the fact that he was recuperating after surgery (Itachi's eye implant).
Pretty sure if you're used to drawing a character for a few hundred characters, you're not going to just forget him. It's just that there's nothing happening for Yamato right now. No one's coming to rescue him, he can't rescue himself, and let's face it...there are other characters who are more worthy of panel time right now.
We haven't even seen anything about Kurenai and her baby since the war started, and they're actually conscious.