```ts
export function flatten(items: readonly unknown[], depth: number = Infinity): unknown[] {
    if (typeof depth !== 'number' || 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[]);
}
```