The law of Unintended Consequences applies....
Free speech gives you rights, it's true. But that doesn't mean you can yell "Fire" in a movie theater when there is no fire. I don't think a warning like you suggest will work as it will be buried the clutter of most websites and ignored by users.
In this case, Section 230 probably seemed like a good idea at the time. Maybe it still is. Who decides what can be published and what can't be? Where is the line drawn?
Maybe a modification is needed such that things be treated on a case by case basis but then... everyone will find something to complain about. Part of the problem is, quite honestly, I believe lawyers. The neverending "you can get compensation for......" ads. Followed by lawsuits for the most trivial of things.