The butterfly keyboard issues are still a problem for those models.

Yes. If you’re looking at a used 2018-2019 MacBook Air, assume the keyboard will eventually fail. Apple’s butterfly keyboards from that era had a well-documented design flaw: dust and crumbs would get under the keys and cause them to stick or stop registering. Apple did a repair program, but it only replaced the keyboard with the same flawed design. They finally admitted defeat with the 2020 MacBook Air that brought back the scissor-switch keyboard.

So if you’re buying used, either budget for a potential repair or skip those years entirely. The 2020 M1 MacBook Air has a much better keyboard and is a better deal in every way.