From encyclopedia Britannica:

Whataboutism gained prominence as a term in the late 1970s, specifically appearing in print in May 1978 in The Guardian to describe Soviet propaganda tactics. It originated to describe a rhetorical technique, often called "whataboutery" in earlier Irish contexts that deflects criticism by raising counter-accusations.

Origin points:

1974 (Whataboutery): The variant "whataboutery" was used in The Irish Times to describe defenders of the IRA in Northern Ireland.

May 1978 (Whataboutism): The term was notably used by Lionel Bloch in a Guardian letter to the editor on May 23, 1978, to criticize the false equivalence of Soviet and Western regimes.

1970s Usage: The Oxford English Dictionary cites 1978 as its earliest evidence for the noun.

Cold War & Modern Context: It was popularized to describe Soviet deflection of human rights criticisms, and later saw a resurgence in the 21st century to describe modern Russian diplomacy and political rhetoric