For anyone else who's interested, they've written/talked about this a few times over the years, to fill out the picture:
- http://amberonrails.com/move-fast-dont-break-your-api/
- https://www.heavybit.com/library/video/move-fast-dont-break-...
- https://speakerdeck.com/apistrat/api-versioning-at-stripe
- https://brandur.org/api-upgrades
- https://news.ycombinator.com/item?id=13708927
It sounds like their YAML system has changed to be implemented in code instead, which maybe allows the transforms to be a bit more helpful/encapsulated. If anyone from Stripe is here, it would be awesome to know if that's true and why the switch?