It seems like Amazon oneboxing in chat is broken completely at the moment. Even tried links that previously oneboxed that don't onebox any more.
Here are some test links:
https://chat.stackexchange.com/transcript/1?m=43501938#43501938
This is another defect on images broken in the onebox from 2017, but this current issue seems to be larger, and not the same.
AWSECommerceService
doesn't love us at the moment, and is 400-ing us; this might be because we have failed to update to their current APIs, but until we can find time to dig deeper than "yup, definitely failing with a 400" it is hard to say.