This is confirmation bias at its finest.
1. "speaking to a few labor lawyers and ran some Lexis Nexis searches" is hardly what I would call conclusive.
2. Companies have been generally instructed not to give feedback in the last few decades, so yes the chances of it happening are now quite low.
3. People don't sue for "they gave me constructive feedback", they use constructive feedback as evidence for some other more legitimate legal reason.
4. Many of these cases get settled (no employer wants to fight them), so you won't find them in public records.
The point isn't that people get sued for providing feedback, it's that the feedback can be used as evidence to build a case.
Look, NO ONE in an org wants to be the person that brought on a lawsuit to the company, so even if orgs don't want to do it, an individual person doesn't want to be that person even if the risk is technically low.