Google Can Ignore All Links on Site If They Detect a Lot of Spam Follow UGC Linkshttps://youtu.be
In this video, John from Google says that if Google sees a lot of follow user-generated links on a site & it believes the site owner isn’t aware of what’s happening. Google’s algorithms can learn to ignore all links on the site to be on the safe side.
This is not something most sites would have to worry about buy say you have a site with a huge user-generated content (UGC) section where spammers are quite actively building outbound links and you don’t nofollow them. There is a chance that Google might think that the webmaster is not aware of what is going on and will choose to ignore links from the site and if they can’t effectively figure out which sections are UGC they may choose to ignore all links from the site ( not sure if internal links will be ignored as well).