During Friday’s Google SEO Office-Hours (December 10 Edition), John Mueller was asked –
"Would I be penalized if I don't set the rel=sponsored or rel=nofollow for my affiliate links?"
His answer was that, Google won’t penalize or give your site a manual action if you do miss to add a nofollow or rel=sponsored attribute to your affiliate links.
According to him, from their point of view, affiliate links fall into the category of financial attached links so they really strongly recommend it. But for the most part, if it does not come across as you are selling links, then it is not going to be the case that they will manually penalize your site for having affiliate links and not marking them up.
However, it is best practice and he strongly recommends doing it, but it is not something where they will go off and manually take action on the sites.
Google already treats most affiliate links as nofollow links even if they do not have the nofollow link attribute to them, including Amazon affiliate links, so this should not be a cause for concern.
Something that the SIA tested though is the use of the rel=sponsored links. It was tested if adding the rel=sponsored attribute to your links can still pass juice or not. Curious what the findings are for this particular tests? Check out seointel.com