Any platform that extracts money from X has the incentive to get rid of any negative reviews about X. Reviewers are not the revenue source. That's why AirBnB, Yelp, Glassdoor, etc will get rid of bad reviews for the right price. Read all bad stuff about AirBnB on reddit, you see where airbnb is heading. People should stop using that platform, whenever you find an alternative.
AFAIK, Yelp has not and does not remove negative reviews for the sake of improving a business's score, but they will remove "spam" reviews (like if a celebrity dies at a hospital and their fans leave bad reviews). There certainly have been minor incidents where certain employees have behaved against policy. Unlike Airbnb, if users lose trust in their reviews, Yelp loses their entire business. source: I am ex-yelper.
Even though whatever Yelp does is not illegal, it engages in unethical practices: "[R]emoving positive reviews wasn’t extortion because Yelp didn’t have to publish those reviews at all. Third, publishing or showcasing negative reviews wasn’t extortion because Yelp has the legal right “to post and sequence the reviews.” Finally, the plaintiffs claimed Yelp wrote bogus reviews to punish non-advertisers, but the plaintiffs didn’t provide adequate evidence that Yelp wrote those negative reviews instead of someone else. For example, even if a business couldn’t find records matching the reviewer’s details, that doesn’t mean Yelp falsified that review (compare the Yelp v. Hadeed case, which in a different legal context gave more benefit of the doubt to an aggrieved business with similar claims)."