Yes, it is completely free, always will be. You can upgrade to a pro plan only if you want to attach your own domain.
Is my data used to train AI models or sold to third parties?
No. Your data is yours.
Do I need to have an account?
Account is not required to publish pages with Folder, however if you want more features like form builder, uploading images etc, you can create free folder.
Any future development plans?
Yes, lots of things on the list, however everything depends on your feedback so list might change. You can check the roadmap page and send feature requests.