diff --git a/ext/etc/extconf.rb b/ext/etc/extconf.rb
index 6e7810a5e8..159b1614b7 100644
--- a/ext/etc/extconf.rb
+++ b/ext/etc/extconf.rb
@@ -47,6 +47,7 @@ if !File.exist?("#{srcdir}/depend")
   %x[#{RbConfig.ruby} #{srcdir}/mkconstants.rb -o #{srcdir}/constdefs.h]
 end
 
+# TODO: remove when dropping 2.7 support, as exported since 3.0
 have_func('rb_deprecate_constant(Qnil, "None")')
 
 $distcleanfiles << "constdefs.h"