Should I avoid buying a used 2018-2019 MacBook Air because of the butterfly keyboard issues?

Yes. If you’re looking at a used 2018-2019 MacBook Air, assume the keyboard will eventually fail due to the dust-and-crumbs design flaw. Apple’s repair program only replaced it with the same flawed design. Either budget for a repair or skip those years entirely—the 2020 M1 MacBook Air has a much better scissor-switch keyboard and is a better deal in every way.