Board Game Arena

Sometimes, I want to let the person know why they got a thumbs-down, to help them get fewer of them in the future.

However, in every single instance, telling them outright or leaving them a comment or message will guarantee me a thumbs-down in return.

Would be nice to be able to leave anonymous messages for people, so they know why they're getting flagged but not by whom.

bad idea.
You assume what you have to say or not.
I think it's better to know who gives you a thumb (even it's not currently done)
Sometimes you receive a thumb down only because you lose (some players are like that, I thnink they use this as marker to see if they already won against you...)...

I'm afraid trying to change someone while giving a thumb down is always doomed to failure.
Either skip evaluation and talk about what bothered you, or sanction an obviously bad behavior with a thumb down and move on?