rel=ugc

Comments, forum posts, profile fields and reviews are the cases it was made for. It tells search engines the link came from someone you do not vouch for, which protects you from what a spammer drops in a comment box.

Most platforms handle it already. WordPress adds rel="ugc nofollow" to comment links by default, so unless you have deliberately changed that, the attribute is on and there is nothing to do. Check by viewing source on a comment rather than assuming either way.

If you cannot tell whether something is user-generated or sponsored, it does not matter much — both keep the link from counting as an endorsement, and Google has said as much. The mistake that costs you is leaving a submitted link with no attribute, because a spammed page with thousands of followed outbound links is a pattern that draws attention.