shadow-cljs does not use CLJSJS sources but if a CLJSJS package provided :externs via deps.cljs these would still get picked up always. This was undocumented behavior and could lead to issues if those externs were written incorrectly.
2.3.0 will no longer include these by default and they are not opt-in only. Via the :build-options :cljsjs-externs key.
{:builds
{:app
{:target :browser
...
:build-options {:cljsjs-externs true}}}
It should not affect many builds but may potentially break release builds that had many CLJSJS dependencies on the classpath. If you run into issues with 2.3.0 I recommend running with :infer-externs :auto rather than relying on CLJSJS externs.