They can also see that you use Discord, as well as how often and how heavily. And potentially other facets that can be derived from whatever metadata is provide

They can also see that you use Discord, as well as how often and how heavily. And potentially other facets that can be derived from whatever metadata is provided to Facebook in the course of serving that image request. There's also all kinds of shenanigans that they can play in the process of serving that request to harvest other meta data and help fingerprint you. Which Giphy's privacy policy mentions is already done to an extent, in the form of dropping cookies while servicing your request. Cookie abuse itself is a bit of a losing battle, as browser vendors increasingly layer on limitations and restrictions for cookies. But they're far from the only method of fingerprinting possible during the servicing of a web request.