Users can now create a reference to a draft within the context of the referring document. This allows for a smoother workflow where having to leave the document you are working on in order to create and publish the document you wish to refer to is no longer necessary,
This new behavior of the reference input can be disabled by setting the property disableNew: true.

More:

• Reference schema type documentation
• Connected Content – Introduction to references in Sanity
  

The ability to define if a field is editable based on conditions defined in a schema.

In addition to being able to provide validation errors, developers can also provide guidance with ℹ️ -symbols to give in-line help to editors.

The Portable Text editor now supports a new modal option with a type and width property. This means you can customize the size of annotation popovers (a much requested feature).

type: 'block',
  marks: {
    annotations: [
      {
        type: 'object',
        name: 'link',
        title: 'Link',
        options: {
          // 👇👇👇
          modal: {
            type: 'popover',
            width: 'medium' // the default is "small"
          },
          // 👆👆👆
        },
      },
    ],
  },
  // ...

• Fixes a case where the sanity graphql deploy command would not exit after a successful deploy.
• Fixes a bug where importing the @sanity/eventsource module in non-browser, non-node.js environments could potentially crash because of a window reference.