I suggest to keep it, maybe rephrase a little that the PR author should squash at the end of the review in case there are to many iteration commits or "fixups" or so. When done only once at the end, the force-push shouldn't be so bad and the reviewer needs to diff on the diff only once (as @b4n said).
So in the end the current rule keeps recommendation but it's not a must. And it depends on PR itself anyway.

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