Steam already does stuff similar to this? Reviews flagged as “suspicious” aren’t deleted but aren’t included in the overall score instead, and a notice is put next to the overall score when this happens. The same is true for reviews made by people who got the game with a key.

Why not extend this? Like you said, most bots have low play time, valve could exclude (but not delete) reviews with low play time. I agree that doing something like this is a slippery slope towards mobile app store reviews but if it’s done right then it is a net positive.