Google announced that it will not be treating the rel="nofollow" attribute as a signal any more, but as a hint. The background is that so many CMS’es are designating every link as nofollow.
They’re introducing two new hints: ugc for user generated content and sponsored for paid links.
For the UGC, they’re explicitly mentioning forums.
My opinion is that this is a big step forward, since the vast use of nofollow was killing the idea behind PageRank.
Can I use more than one rel value on a link?
Yes, you can use more than one rel value on a link. For example, rel=“ugc sponsored” is a perfectly valid attribute which hints that the link came from user-generated content and is sponsored. It’s also valid to use nofollow with the new attributes – such as rel=“nofollow ugc” – if you wish to be backwards-compatible with services that don’t support the new attributes.