I've only seen them when paying for parking in the EU. They do work as the poster said.

I have used them once to pay in a restaurant in Prague.

There was an App Clip code on the table that you could scan to pay.

The UI/UX was pretty nice, but I bet you could make a website also look that nice and work as well.

I have also seen them around Japan, but of course, Japan being Japan, if you scan them you’ll normally get an error saying that it’s not supported on your non-Japanese-region set phone.

For instance, Starbucks Japan has them everywhere: https://atadistance.net/2024/05/22/starbucks-app-clips-an-ap...

Toast used it pretty effectively for QR code-initiated restaurant bill payments. Though in the US I've only seen a few restaurants trust customers to pay by QR code and not physically want to handle a card.

Though the other notable thing about it was that the App Clip version does a decent upsell to get you to install the full Toast Local app. Then once you have the Toast Local app you never see the App Clip version again. So there's probably other things that have App Clips but you would not notice because you already have the full app.