I think it is better not to introduce them to the proposal system. Not only because they are very controversial but also because they can be obtained in a different way.

Unlike the reward system for posts, the SPS does not have manabar or limit in the number of votes. So, if you want to penalize a particular proposal you can give votes to other proposals, the effect will be the same as a downvote. And, as I said, there is no restriction in the number of votes. It is like increasing the threshold for certain proposals.