And people downvote you as a disagreement button on this site instead of qualifying their opinion in words.

But you’re not wrong in your opinion and allowing downvotes without requiring a response encourages groupthink as well.

But where’s the line? Do we as society mandate freedom of speech on all private platforms? Are downvotes on HN protected speech?

You’re right that it’s a tough problem. We are at an inflection point right now. The boundaries of speech freedoms for the next century are being defined right now.