The maintainer of the feature here.
If you want to use protocol version "1.1" you can go ahead and do it. Chromium does NOT hide protocol methods specific to 1.1 if you requested 1.0, I can see how this can be confusing.
The fact that Chromium currently does not accept "1.1" as the protocol version in thechrome.debugger.attach
call is a bug.
Thanks for pointing this out, I am fixing it now.