Google's Gemini API terms permit human reviewers to read free-tier inputs and outputs, and instruct developers not to submit personal information to the unpaid tier.
Anthropic and OpenAI do not train on API data by default, on free or paid keys, which makes them the safer default for anything touching user data.
On OpenRouter, opting out of provider training shrinks the pool of free models you can route to, because many free endpoints require training to be enabled.