@automattic/calypso-products/src/is-custom-design.ts

This module contains information about the various products sold within calypso, such as product slugs, plan intervals, etc.

import { camelOrSnakeSlug } from './camel-or-snake-slug';
import type { WithCamelCaseSlug, WithSnakeCaseSlug } from './types';

export function isCustomDesign( product: WithCamelCaseSlug | WithSnakeCaseSlug ): boolean {
	return 'custom-design' === camelOrSnakeSlug( product );
}