There is nothing stopping you from making an impure function, but the language naturally guides you towards writing code that's composed of a bunch of functions that transform data. Clojure also has explicit containers for mutable data and those have their own semantics, so you can't accidentally cause mutation.

It's also worth distinguishing between different kinds of side effects. One type of effects is doing things like logging, which in my experience are generally harmless. The type of side effects that I tend to worry about are the ones that couple functions together via mutable references.

And yeah, Typed Clojure library lets you do gradual typing, but it hasn't really caught on with the community so far. Most people prefer using runtime contracts with Spec and Malli.