Is CLOS still worth learning?
Well, it is integrated into Common Lisp, so if you want to master Common Lisp and read other people's Common Lisp code, you should learn CLOS. Some important libraries such as the Hunchentoot web server (used by this website's pastebin) are written using many CLOS features.
https://en.wikipedia.org/wiki/Common_Lisp_Object_System#Literature has a good list of resources for learning CLOS. The typically recommended book for an introduction to CLOS is Object-Oriented Programming in Common Lisp: A Programmer's Guide to CLOS by Sonya E. Keene.
https://en.wikipedia.org/wiki/Object-Oriented_Programming_in_Common_Lisp