```ts
export function flatten(items: readonly unknown[], depth?: number): unknown[] {
  if (!Number.isInteger(depth) && depth !== Infinity) {
    throw new Error("Depth must be a non-negative integer or Infinity.");
  }

  const recursiveFlatten = (item: unknown, currentDepth: number): unknown[] => {
    if (typeof item === "object" && !Array.isArray(item)) return [];
    if (currentDepth >= depth) return [item];
    if (Array.isArray(item)) {
      return item.flatMap((subItem) =>
        recursiveFlatten(subItem, currentDepth + 1)
      );
    }
    return [item];
  };

  return items.flatMap((item) => recursiveFlatten(item, 0));
}
```