Proper way to enforce a single document type in Sanity schema, with deprecation warning for deskTool.

import { defineConfig } from 'sanity'
import { structureTool } from 'sanity/structure'

// Define the actions that should be available for singleton documents
const singletonActions = new Set(['publish', 'discardChanges', 'restore'])

// Define the singleton document types
const singletonTypes = new Set(['authorSettings'])

export default defineConfig({
  // ... other config
  plugins: [
    structureTool({
      structure: (S) =>
        S.list()
          .title('Content')
          .items([
            // Your singleton document
            S.listItem()
              .title('Author Settings')
              .id('authorSettings')
              .child(
                S.document()
                  .schemaType('authorSettings')
                  .documentId('authorSettings')
              ),
            // Separator
            S.divider(),
            // All other document types
            ...S.documentTypeListItems().filter(
              (listItem) => !singletonTypes.has(listItem.getId())
            ),
          ]),
    }),
  ],

  schema: {
    types: [
      // your schema types including authorSettings
    ],
    // Filter out singleton types from the global "New document" menu
    templates: (templates) =>
      templates.filter(({ schemaType }) => !singletonTypes.has(schemaType)),
  },

  document: {
    // For singleton types, filter out actions that are not explicitly included
    actions: (input, context) =>
      singletonTypes.has(context.schemaType)
        ? input.filter(({ action }) => action && singletonActions.has(action))
        : input,
  },
})