As I was going through the tag-wikis today to try to add excerpts to those that are lacking, I found many plagiarized tag-wikis and excerpts.
The listed examples is just a drop in the ocean. There are far too many of them to be listed here.
What I mean by plagiarism is that the content has been copied from somewhere else (usually Wikipedia) but has not been attributed at all.
Considering that we have a strict policy against plagiarism in answers, I think we would require one for tag-wikis as well.
While I could have just added in the reference links to the plagiarized content, I don't think this would send the right signal to inadvertent offenders. But, adding in the reference links is the only stopgap solution that we have in our hands until we manage to do something about this.