As the person that wrote such documentation, I respectfully disagree, I understand you point but I want to tell you why I believe the way it is, is better for Redis. Redis is a system that is 99% backward compatible across all versions combinations: Redis 2 was released more than 10 years ago, and this is a very hard to find case where things are not perfectly backward compatible, but still in a case where the Redis 2 semantic was so odd to be avoided in most tasks. Now, in a system like that, to have man pages that tell you the differences among versions is much better than versioned documents where you would require to diff yourself among the different pages. In a single document you know all the behavioral history of a particular command, that often is just: "always as it used to be", plus features that are backward compatible entering newer versions.
I think a changelog on the page is key, as you say. I also think that having versioned docs does not requires us as users to do diffs manually, that's what a changelog/history is for. Ideally, I'd like to have both: all docs are versioned and all docs have "history" sections.