Unsupported "managed" storage type

[DamienCassou]

There doesn't seem to be any "managed" property on browser.storage. The code says it is "unsupported":

declare namespace browser.storage {
  "...";
  const sync: StorageArea;
  const local: StorageArea;
  // unsupported: const managed: StorageArea;

  type ChangeDict = { [field: string]: StorageChange };
  type StorageName = "sync" | "local" /* |"managed" */;
}

Why is "managed" not supported?

For reference, here is the error message I get when trying to use it:

[tsl] ERROR in /.../src/background.ts(61,48)
      TS2339: Property 'managed' does not exist on type 'typeof storage'.