All of your data and projects will still be accessible once the account is reverted to a free account.
Some paid services like Landing Pages and Forms may cease functioning if your free subscription doesn't cover your existing projects.
The service is entirely based on the subscription, so if the permissions to use those specific channels are disabled, they simply won't work. After all, you wouldn't keep allowing customers to connect to the internet if they stopped paying the bill. This is just like any other service.