It would help reduce backup sizes on macOS for Time Machine and 3rd party backup tools if Shadow's generated files, caches, and target directories were marked to not be backed up. There is a command that can check any path for you to see if it is backed up or not:

$ tmutil isexcluded .shadow-cljs/
[Included]  [...]/.shadow-cljs
$ tmutil isexcluded /Users/<myuser>/Developer/clojure/clj-rethinkdb/target
[Included]    /Users/<myuser>/Developer/clojure/clj-rethinkdb/target

Apple provides an API CSBackupSetItemExcluded to mark directories as excluded by Time Machine, and other Mac backup software respects the same markings. There is also a command line tool tmutil which lets you accomplish the same task with tmutil addexclusion <file path>.

Relates to boot-clj/boot#628, technomancy/leiningen#2292.

IMO generated directories shouldn't be backed up by default, though I could see this being a user setting (not a per-project setting).