So I looked it up. Only to find that its first usage was as a line around a military prison beyond which a prisoner is liable to be shot.

_____________________________<1864 “The ‘dead line’, beyond which the prisoners are not allowed to pass.”—in Congressional Record, 12 January (1876), page 384/1>
<1868 “ Seventeen feet from the inner stockade was the ‘dead-line’, over which no man could pass and live.”—History Civil War U.S. III by B.J. Lossing, page 600>
<1888 “Should he some day escape alive across the dead-line of Winchesters, he will be hunted with bloodhounds.”—The Contemporary Review, March, page 449>
<1889 “The instant he sought to cross the social dead-line.”— The Plantation Negro as a Freeman. Observations on his character, condition and prospects in Virginia by P. A. Bruce, page 45>
<1920 “Corinne Griffith . . . is working on ‘Deadline at Eleven’, the newspaper play.”—Chicago Herald & Examiner, 2 January, page 10/4>
<1929 “Deadline for Poetry's $250 prize poem contest is September 1.”—Publisher’s Weekly, 27 July, page 349>
<1948 “The Security Council will not meet again until Wednesday, about 20 hours after the dead-line.—Daily Telegraph (London), 31 May, page 6/5>
<1982 “Applicants must have experience in office and business management, preferably in a school. Please send resumes to Box P243 Globe Office, Boston Globe, Boston, MA 02107. Deadline date Aug. 11.”—The Boston Globe (Boston, Massachusetts), 1 August, page 166>
<2019 “The relationship is likely to become more significant as a deadline nears for India to comply with U.S. sanctions against Iran, one of India’s main oil providers.”—The Seattle Times (Seattle, Washington), 20 February>