Welcome to your interview.
The question is on your right: Ship Backward-Compatible Mobile Features. Take a moment with it first.
Talk your thinking through with me if you like - when you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes). Discussion and graded submissions share your five interviewer interactions, so spend them well.
You are preparing to ship a new mobile feature to a large installed base where many users stay on older app versions for weeks or months. The feature depends on new client behavior, but your backend and product experience still need to work for users who have not updated yet.
How do you think about backward compatibility when shipping mobile features to a large installed base?