> the problem that now reviewers have to figure out what changed
That's something that the review tool can solve, and I agree that github doesn't handle it well. But other code review tools can show diffs between revisions independently of how the commits have been rebased or squashed over time.
That's part of the problem, if you are a reviewer you'll see the changes that other reviewers asked for on code that you did not want to review.
I guess this might be a problem mostly on large projects on monorepos, where different systems that interact end in the same codebase, but there's a cost to be paid too if each system has their own repo, build/test/deploy infra and independent ownership.
That's something that the review tool can solve, and I agree that github doesn't handle it well. But other code review tools can show diffs between revisions independently of how the commits have been rebased or squashed over time.