Most serious Lisp programmers write mutable code. Common Lisp is multi-paradigm but most code written has side-effects. And that’s for a good reason: Clojure-style immutable programming doesn’t fit most problems very well. Which is why Clojure will never be a language with the lasting power of Common Lisp. It prematurely optimizes and constrains your thinking into a single paradigm.
I write CL production code all the time. Basically daily. I use hash tables a lot, and those are obviously mutable (I did mention that as an exception) but most of the rest of my code is pretty much functional, which is generally (though not uniquely) immutable. I can’t really see how it makes sense to do hash table immutably. It might just be me, but functional style tends toward immutability be default.
