Too Long; Didn't Read
Aakash Khatu created a Chrome extension that allows you to restore deleted messages on WhatsApp web. The extension stores all messages it receives locally. When a message gets deleted, it adds the previously known message contents to a deleted messages array. Then it replaces the “This message was deleted ” text with its previous contents. Once the message gets restored you can even quote it in a reply. Using this on non deleted messages only edits the message for you. Sadly, the code from the extension is obfuscated and difficult to read.