No, you won't lose your flows when downgrading from Pro to the free plan. When you cancel or downgrade your plan, your account data (including tables and flows) remains intact.
Your workspace will be moved to the free plan at the end of your current billing cycle, and you'll still have access to your workspace and any tables you've created.
However, you'll lose access to Pro-exclusive features, and your credit balance will be capped at 200 credits (2x the free plan's monthly limit).