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

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