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That’s literally the point of the downvote system. To downvote posts you don’t like, or you feel are out of place.
This seems to be the real issue you’re trying to fight. It seems like only permitting downvotes on communities that the user has been a part of for greater than 1/2/7/30/pick-a-number days would be the proper solution. If people in a community are downvoting a post, then it means they don’t think that post is worth sharing. No admin, moderator, community owner, etc. should be able to change that.
I am strongly against removing downvotes.