I got really excited about this thinking that it was something that worked outside of the code base as an interface proxy that could work cross-language.
Yes, I was imagining a "traffic demultiplexer" of some sort that could feed the old and new backend in parallel and observe they have the same behaviour.
I think something like that would be useful for a lot of people in industry who have to maintain legacy monoliths. The ideal tool would allow "switchover" from old to new gradually, one URL endpoint at a time.
This can be used to replay traffic https://github.com/buger/gor/ so can do load testing at least...