_Comment made by: mfikes_
I know David suggested that a hard error is probably the right thing to do for this one, but one consequence is that the {{cljs.spec/def}} macro cannot be defined in bootstrap with this change. (I haven't investigated thoroughly, but this may simply be the result of macros being processed as ClojureScript in bootstrap, and thus being subject to this new guard.)
Regardless of the root cause, you'll see this if you try to run {{script/test-self-parity}}:
#error {:message "Could not eval cljs.spec", :data {:tag :cljs/analysis-error}, :cause #error {:message "Can't def special form at line 51 ", :data {:file nil, :line 51, :column 1, :tag :cljs/analysis-error}}}
For reference: Line 51 currently points at the {{def}} macro:
https://github.com/clojure/clojurescript/blob/e2db5d9ff8cb6a099ebc2a8cd379385bf4649b38/src/main/cljs/cljs/spec.cljc#L51