In John Mueller’s hangout on 09/10/2021, one webmaster asked about affiliate backlinks. Should they be using nofollow on affiliate links? If your users are the people linking to you, and they are following those guidelines, John thinks that’s fine.
You don’t want to disallow crawling of the affiliate parameters, however. Doing this will result in those pages being indexed without any content.
John recommends focusing on the normal canonicalization process.
He explains that he has seen some sites set up an entirely separate affiliate domain, which is blocked from crawling and indexing. He has also seen the variation where one is blocked from crawling and is then redirected to the website.
He believes that for very large-scale affiliate websites, this could make sense. It is, however, probably overkill for the average site.
This discussion occurs at approximately 26:17 in the video: