I live in the UK, and I don’t think I’ve used my mag stripe here once in the last decade. The only time I can remember using it was on a plane.

Even EMV contactless is designed to work offline. There's no need for the mag stripe at all anymore unless you encounter a truly ancient card terminal. The last time I remember encountering one was at a toll booth in Italy around 2011 or so. And the transaction was immediately declined by my UK bank!

I've always wondered: What happens if I use a paused/canceled card on a plane? Do I just get the item for free?

Yep, like this person who used that method to steal £25,000 worth of goods from airlines: https://www.manchestereveningnews.co.uk/news/greater-manches...

Transport for London works similarly for contactless payment card acceptance on the tube, buses, etc. Cards are authorised in offline mode, with transactions processed overnight at the end of each day.

But they also maintain an internal blacklist of declined cards, so the most you'll get away with is one day's travel around London. Per card.

Oh wow, that's taking things to another level.