While the discussion seems to have expanded beyond just site-wide technological solutions (to detecting, catching and discussing larger cases of SEO / link spam),
rel="ugc" attributes on links posted by Newbies (so essentially anyone without any merit points) are already being added since around January, 2021:
Not sure what they're trying to do. In case it is some SEO-motivated thing, I added rel="ugc" to all newbie links. While I was at it, I also added this to all links in signatures.
Thanks to everyone reporting them!
Someone did notice the
rel="ugc" attributes on signature links but I haven't seen the change to newbie links mentioned here (hence this post). Nevertheless, do keep reporting any cases of SEO spam you notice - especially if posted by non-newbie users.