And how representative are publicly accessible redis/valkey instances for redis/valkey usage in general? And can shodan even differentiate Redis from a Valkey instance setup in a backwards-compatible way without being able to authenticate?

In absolute numbers probably not highly representative but the relative numbers are meaningful to measure adoption. And no, it requires the user to disable authentication in order to get the service details to differentiate between Redis and Valkey. But again, you can compare unauthenticated Redis to unauthenticated Valkey to see how the percentages are changing over time.