Welcome to the Auth0 Community!
Allow me to try and provide some clarity with regards to your current use-case.
Firstly, while you are on a Free plan, you will not get charged for using the Paid features. As the warning states, to continue using them you will need to upgrade your subscription, but this will not happen automatically.
Secondly, using the Paid features while on a Free plan can result in you getting blocked if you do not upgrade your subscription. Adding to this, exceeding your MAU limit can again lead to you being blocked.
To rest assured, you can either stop using any Paid features or your can upgrade your subscription to include the features that you require. You can view mode information on our Manage Subscriptions documentation. Access to the paid features on Free tenants is more or less intended for testing purposes, to ensure if the features fulfil your needs before making any purchases.
Hope this helped!
Gerald