There are also many changes in the language itself (see https://github.com/clojure/clojure/blob/master/changes.md https://github.com/clojure/clojure/blob/master

There are also many changes in the language itself (see https://github.com/clojure/clojure/blob/master/changes.md https://github.com/clojure/clojure/blob/master/changes.md), none of which are alpha. The things that are really the new big drivers are pieces that are not inside the language (other than a few spec integration points). Those things are significant pieces of work that are imminently usable now, but not quite final yet. In the interest of getting this work into more hands, we made a new release of the language with those parts marked as alpha. The modularized spec allows us to continue releasing new versions (and for you to use them) with Clojure 1.9.0. Eventually spec will leave alpha and we will consider that a finalized part of the language. Similarly, tools.deps is still evolving as we work on it but the tools should remain fairly stable.