Do affiliate links need to have a rel=?nofollow? attached in the

For affiliate links on sites, do you need to add the rel="nofollow or rel="sponsored" attribute? I heard that Google is pretty lax on this and doesn't always require it. But, if it is a requirement, I will do it. I just want to know, because the link looks weird. I could still use a hyperlink though, or if I need to a link shortener.

Anyway, what happens if you don't add this to your affiliate links?