The Short Version
The back-pass rule was formally adopted by IFAB at its 1992 annual general meeting and became a binding Law of the Game effective July 1, 1992. Multiple independent and credible sources confirm this date. While a limited experimental trial took place at the 1991 U-17 World Cup, that was a single-tournament test—not a universal rule change. The standard understanding of "introduced" aligns squarely with 1992.