When were air filled bags first introduced on the landing side of the high-jump bar?

They were first used at the Olympics in 1968. They were used at U.S. high schools before that.

High Jump -- Over the centuries, high jumpers have used many different techniques to get over the bar including the scissors, feet first, western role and straddle. The flop, a backwards jump, that was introduced in 1968 (with the introduction of a thick cushioned landing mattress), has been used by all the top jumpers since 1978.
2001.edmonton.com/index.asp?l=en and p=320 no longer exists

Up until the 1960s, high jump athletes cleared the bar and landed on hard ground—sawdust, sand and low mats. As a result, innovation of the high jump techniques attempted to ensure that the athletes landed on their feet. Fortunately for Dick Fosbury, his high school was one of the first to install a deep foam matting for high jump landing.This new environmental shift gave Fosbury the opportunity to try out new ways to clear the bar i.e. Landing on his back instead of his leg.

Fosbury wasn’t the only person to innovate new ways of clearing the bar. Around this same period, a Canadian teenager, Debbie Brill, also decided to experiment with new ways of clearing the bar, after the foam landing mat was introduced to her high school.

https://mayooshin.com/dick-fosbury/
1968 Olympics gold: https://www.youtube.com/watch?v=9SlVLyNixqU
Short documentary: https://www.youtube.com/watch?v=CZsH46Ek2ao