Is using a modified WhatsApp version developed by unofficial parties and unauthorized by the parent company considered a sin, knowing that it does not contain ads and does not harm the company?

It is not permissible to download modified applications due to the violation of intellectual property rights therein, for intellectual property rights are protected for their owners. The fact that these applications are free or ad-free does not justify downloading them, nor is the claim that no harm befalls the program owners valid. Some scholars, however, permit the use of pirated software for personal benefit.