Kapps:
Flags such as -O are specifically not supposed to change program behaviour. This being the case would completely discard that and allow code to be compiled only with a single compiler.
OK.
Honestly, I think expecting that code to be allowed to use @nogc is a huge mistake and disagree with editing the DIP to include this solely because you decided it should.
That Wiki page is editable, so if it's wrong it takes one minute to fix the text I have written. What I have decided to include is an explicit explanation regarding what a correct D compiler should do in that case.
Bye, bearophile
