When was the butterfly keyboard introduced to the MacBook Air?
I assume you're trying to avoid purchasing a model with a keyboard that is prone to errors. Apple's "Keyboard Service Program" for the MacBook Air lists the MacBook Air Late 2018 and Mid 2019 models as eligible.

The Mid 2017 MacBook Air and older have the old scissor-style keyboard.

The Late 2018 MacBook Air has the new butterfly style keyboard. It has the newer version of the butterfly style keyboard that has a membrane to keep dust from jamming the keys (as compared to older MacBook Pros that featured a butterfly style keyboard without that membrane). So does the Mid 2019 model.