First of all, it should be noted that a Muslim is demanded to be kind and merciful even in dealing with animals. It was authentically reported that a woman was admitted to Hellfire because she deprived a cat from eating and drinking until it starved. On the other hand, the Prophet (peace and blessings be upon him) told us that a man entered Paradise because he brought water to a thirsty dog.
Dr. Sano Koutoub Moustapha, professor of fiqh and its principles, International Islamic University, Malaysia, states: Islam calls upon Muslims to fear Allah in everything including the animals. Islam also calls upon Muslims to be gentle and nice to everything and at all times even when they want to slaughter animals.

In this context, declawing a cat is a form of harming, oppressing, and depriving it from its rights of having claws, which it needs not to scratch your furniture but to scratch and defend itself.