Maintenance free batteries hit the market in 1970 (just googled it). 1970 was a little more than 20 years ago by well over twice. That the alloy sat there for 20 years means little, the metals used to make the alloy aren't going to evaporate and are still in the alloy. The combination of the components in the alloy and the percentages of them when melted is what produces the gases that are dangerous.
Rick