They are still useful because they identify fundamental patterns in how code gets organized and different concepts get connected. GoF design patterns give you a linguistic description of common structural organizations of data/logic flows.

You should have no trouble finding it used for single-digit dollars. If you decide to get it, read the first and last chapters. Like, actually read them. They are the chapters that don’t have any design patterns in them, so pretty much nobody reads them. But you should. Whether or not you study any of the design patterns in the book is up to you. They don’t really apply to a lot of things people are doing these days.