Unicode isn't a synonym of UTF-8, but it is a superset. Since UTF-8 is the de facto standard representation of Unicode in the unix world, most questions about Unicode on this site involve UTF-8. While some questions are specifically about UTF-8 and others are about in Unicode in general, we don't seem to be following any rule as to which questions are tagged unicode and which ones are tagged utf-8 (beyond perhaps the rare question about other representations not being tagged utf-8).
I don't see any point in having two separate tags. This only makes it harder to find Unicode-related questions since there are two tags to search. Therefore, I propose to make utf-8 a synonym of unicode. (unicode has to be the main tag since it's more general.)
I can't propose the synonym through the tag interface because “Version specific synonyms can only be created by moderators”.