When I learned (Common) Lisp at Georgia Tech in the summer of 1995, we were encouraged to make as many functions as possible purely functional, and if mutation was needed, to try to hide it within the bounds of a function.

So while Lisp may not be purely functional, the culture hewed that way.

Sure the language might not be immutable but the lisp machine alt universe's "overton window" would I imagine be more functional then today's (JVM? maybe that's the qnalog?) reality.