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

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