```ts
export function flatten(items: readonly unknown[], depth?: number): unknown[] {
  if (depth === undefined) {
    depth = Infinity;
  }
  if (typeof depth !== 'number' || isNaN(depth) || depth < 0) {
    throw new Error('Depth must be a non-negative integer or Infinity.');
  }

  return items.reduce((acc, item) => {
    if (Array.isArray(item) && depth > 0) {
      acc.push(...flatten(item as unknown[], depth - 1));
    } else if (typeof item !== 'string') {
      acc.push(item);
    }
    return acc;
  }, [] as unknown[]);
}
```