Robin’s prophecies have been on point so far, and Fitz has maintained all along that certain aspects of time can’t possibly be changed.
Agents of S.H.I.E.L.D. may very well be planning on killing somebody (or somebodies) off, and “The Last Day” may have dropped some hints about the characters most likely to bite the dust.
This particular prophecy indicates that at least one of the group will die and/or be left behind when the rest of them return to 2017.
Robin revealed to May early in the episode that the last time anybody ever saw the full group — that is, Coulson, May, Daisy, Fitz, Simmons, Mack, and Yo-Yo — together was when they were in the diner right before the crazy Season 4 cliffhanger.