Spoilers ahead for Episode 10 of Agents of S.H.I.E.L.D. Season 7, called “Stolen.”
“Stolen” unfortunately added a lot more weight to the theory that Fitz has actually been dead all along, and Simmons just used Diana to make herself forget.
The episode ended without giving a definitive answer to what happened to him, but it did reveal that Fitz might be the key to everything.
The big mystery of Agents of S.H.I.E.L.D.’s final season has centered on the long-lost Fitz, who may or may not have spent the first part of Season 7 guiding Simmons and the rest of the team through time.