The image above is a sneak peek of the developer portal that will go live tomorrow.
Here's more info from the email they sent me:
The Notion API will be available to all as a public beta starting on May 13th. […]
The API now contains many of the fundamentals: reading and writing to pages, querying databases, authorization, and more. This is enough to build many of the interesting integrations you've told us you want to build. And on May 13th, you're in. API integrations will be available to all Notion users, on free or paid plans.
Our goal is to establish that the Notion API is robust, easy to use, and trustworthy. In public beta, we'll be adding new features and making significant changes based on your feedback. Once the most important improvements are included, the API will transition from public beta to general availability — you'll have everything you need to build integrations teams and businesses can depend on.