The one thing that bothers me about Notion (and Slack and other "everything in one place" tools), is the lack of encryption. I might have FAANGophobia, but whenever there is a free tier without a form of end-to-end encryption in place, it feels like a data puddle waiting to become a lake.
That being said, having clear-text data would allow features like an API on publicly shared pages/blocks, to use Notion as a CMS. I have seen some attempts [1] at reverse-engineering their internal API, but an official one on a paid plan could be a nice addition.
There is no easy way to implement client side encryption. You will have a private key or long password the you will keep safe. You lose that all your data in gone. Plus it's difficult to securely move that password to a new platform