Add deprecation for 2-arg @boundscheck.

base/deprecated.jl

@@ -936,3 +936,11 @@ end
 
 #14335
 @deprecate super(T::DataType) supertype(T)
+
+#14474
+macro boundscheck(blk, truefalse)
+    depwarn("The meaning of `@boundscheck` has changed. It now indicates that the provided code block performs bounds checking, and may be elided when inbounds.", symbol("@boundscheck"))
+    if truefalse === true
+        :(@inbounds $(esc(blk)))
+    end
+end