[Hyperfixed] has a podcast episode that does a deep dive into exactly why bags of microwave popcorn advise against using your microwave’s popcorn setting (in North America, anyway.) The answer is probably more complex than one expects, and involves the long history of microwave oven development as well as consumer behavior.
There are really two questions involved: one is “what does the Popcorn button actually do?” The other is “Why does every package of microwave popcorn say not to use it?”
The answer to the first question is “it depends”. Microwave ovens are amazing things, but the popcorn setting on any given microwave oven could be anything from just a dumb timer, to a setting that tells the oven to monitor popping sounds or humidity levels (because popping releases steam). These kinds of microwave oven features were figured out ages ago, meaning the technology to precisely handle a bag of microwave popcorn not only exists in equipped ovens right now, but has existed for decades.
The problem is not all microwaves are so equipped. A microwave oven where the popcorn setting is just a dumb timer is really an invitation to under- or overcook the bag. And when overcooked, microwave popcorn scorches and could even burn.
That brings us to the answer of the second question: why does every package warn not to use the popcorn setting? In short, because the popcorn maker has no idea what any given user’s microwave oven is doing. When people hit a button marked “Popcorn” they tend to leave the process unattended, and when a microwave oven ruins a bag of popcorn, the user blames the popcorn manufacturer.
This means microwave popcorn manufacturers stick to some variant of the only process known to work in all ovens: monitor the process, wait for popping to slow to a rate of one pop every two seconds, then stop.
That’s the quick answer, but [Hyperfixed] goes in a ton of researched detail across the entire spectrum of this issue. If deep-dive niche technical podcasts are your thing, we recommend queuing it up for a listen.