Sam Witwicky (Shia LaBeouf) was the hero of the first three Transformers movies. He was a scrappy, nerdy kid who came across the Transformers (or rather, the Transformers came across him because the coordinates of the AllSpark were imprinted on the glasses of his great-great-grandfather, Archibald Witwicky). After the third Transformers movie, Sam left the franchise, and there was with no mention of his life or whereabouts in the follow-up, Transformers: Age of Extinction, which followed new hero, Cade Yaeger (Mark Wahlberg).
So what happened to Sam Witwicky? The new Transformers movie, Transformers: The Last Knight, has the answer.
[Spoilers ahead for Transformers: The Last Knight]
If Vivian is the last surviving Witwicky, that means Sam Witwicky is likely dead. How he died and his life post-Transformers: Dark of the Moon is left a mystery, but it’s an ignominious end for a guy who was ostensibly the lead of the first three Transformers movie. Granted, no one expected LaBeouf to ever return, especially with Bay at the helm. That being said, since Sam’s death is glossed over and these movies lack cohesive storytelling anyway, I doubt anyone would bat an eye if Sam just magically reappeared in a future Transformers movie.