The messages highlight the illegality of what people are searching for online and direct them towards help.
The regulator said the forum had not done enough to protect UK users – but critics accuse Ofcom of acting too slowly.
A cyber security expert says deleting chat history could lead to a lack of accountability if things go wrong.
The voluntary recall follows an incident on 20 April where an empty Waymo car entered a flooded road in San Antonio, Texas.
Quantexa, a financial data platform, won the £175m contract to spot fraud and tax return errors.