Tommy only discovers the truth in the series finale after seeing a newspaper clipping of Mosley’s wedding, which featured Dr. Holford as a guest. Why He Didn't Die
However, the reason Tommy is "still alive" by the end of the series is one of the show's biggest twists. It turns out he was never actually terminally ill. The Great Deception [S6E4] Why Is Tommy Still Alive?
As of the latest developments, Tommy Shelby remains alive and is set to return for the upcoming Peaky Blinders feature film, where he must continue his fight against the rising fascist threat. Tommy only discovers the truth in the series
A recurring theme is that Tommy "died" in the tunnels of WWI in 1918, and everything since has been "borrowed time". It turns out he was never actually terminally ill
Mosley used Dr. Holford—a fellow fascist—to convince Tommy he was dying, hoping Tommy would take his own life to avoid a slow deterioration.
Beyond the medical fake-out, Tommy’s survival is also tied to his character's "immortality" through the war.
The diagnosis was a orchestrated by his political rival, Sir Oswald Mosley.