Unclear Ruling, But Certainly Risky
According to Amazon's TOS, "any attempt to manipulate the search and browse experience is prohibited" as Amazon wants customers to only find "relevant and accurate results".
Super URLs have not yet been explicitely forbidden and there have so far been no reportings of accounts being banned because of them.
Recomendation: Only for External Marketing Tracking
Although one could argue other means such as PPC would also count as manipulating the keyword rankings, it is nevertheless recommended to stay on the safe side and only use Super URLs for the tracking of External Marketing activities, rather than to push keyword rankings.
Furthermore it is unclear whether Super URLs are actually still effective (or ever have been), as it is difficult to prove this beyond doubt.