On another post someone mentioned AC 797, 6/2/83 flying DFW-YYZ had a fire in-flight, made an emergency landing CVG 23 passengers killed. AC uses flight 797 YUL-LAX now. Also I found UA 227, 11/11/65 727 was flying LGA-CLE-ORD-DEN-SCL crashed at SLC 43 people killed. UA uses flight 227 now MSP-DEN-LAX. Flying is much more safe then in the 1960's and 1970's when there seemed to be airline accidents on a regular basis.
Not sure of the policy of retiring flight numbers but the airlines much feel after time people would not remember accident flight numbers.