> If you've never used Clojure and start a Clojure project, you will almost definitely find advice telling you to use Leiningen.
I thought the current best practice for Clojure was to use the shiny new built-in tooling? deps.edn or something like that?
deps.edn is becoming the default choice, yes. I interpreted the parent comment as saying "you will see advice to use leiningen (even though newer solutions exist, simply because it _was_ the default choice when the articles were written)"
Clojure CLI (aka deps.edn) came out in 2018 and in the survey "how do you manage your dependencies?" question crossed 50% usage in early 2020. So for 6-8 years now.