What is the ruling on using a feature in a hacked WhatsApp that enables the user to see messages that the sender has deleted, and is this considered spying?

The Sunnah (definition) prohibits listening to someone speak when they dislike being heard. Al-Bukhari narrated from Ibn Abbas (may Allah be pleased with them both) that the Prophet (peace be upon him) said: "Whoever listens to the discourse of a people while they dislike it, or flee from him, molten lead will be poured into his ears on the Day of Resurrection."

In Al-Mawsu'ah Al-Fiqhiyyah (The Fiqh Encyclopedia): "There is no disagreement among jurists that eavesdropping—which is listening to people's conversations without their knowledge or consent—is forbidden, and the eavesdropper will be punished for it in the Hereafter." And it is narrated in Sunan Abi Dawud from Abdullah ibn Abbas (may Allah be pleased with them both) that the Messenger of Allah (peace be upon him) said: "Whoever looks into his brother's book without his permission, he is only looking into the Fire."

This is because it is understood to apply to a book containing secrets and private matters which its owner dislikes being seen. Looking at deleted messages is similar to what has been mentioned previously; the default rule for looking at them is prohibition, unless there is a justification, such as it being a means to prevent harm or similar.

Looking at deleted messages is not considered espionage. Espionage is searching for hidden matters and differs from eavesdropping in that espionage involves digging for specific matters that the spy seeks to obtain, whereas eavesdropping involves merely acquiring information that one happens upon. Allah knows best.