Julia, perhaps? Although calling it popular is a stretch maybe. Its multiple dispatch system is even more ingrained to the language than CLOS (all functions are generic by default). It's very much a Lisp in Matlab's clothing.

Ruby has a meta object protocol. The net effect of the language isn't as powerful as Common Lisp since you don't have code-as-data in method definitions, but Ruby's object system is comparably powerful as CLOS (though made in the image of Smalltalk) it comes with a meta protocol for methods, objects, classes, modules, even lexical variables.

https://ruby-doc.org/3.2.2/Object.html

https://ruby-doc.org/3.2.2/Method.html

https://ruby-doc.org/3.2.2/Class.html

https://ruby-doc.org/3.2.2/Module.html

https://ruby-doc.org/3.2.2/Binding.html

Julia, as answered on the sybling comment.

Clojure has a subset, and I guess Racket is quite flexible with its language modeling framework.

Going on an edge, C++ is also quite flexible when one looks beyond of plain classes, and combines them alongside type parameters in templates, and compile time reflexion via template metaprogramming and concepts (although it is quite clunky).