UserAgent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_11_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/68.0.3440.106 Safari/537.36

Steps to reproduce the problem:
1. Open http://www.example.com/
2. In a chrome extension with debugger permissions run this code:

chrome.debugger.getTargets(function(targets){
   const target = targets.find(t => t.url === "http://www.example.com/")
   const targetArg = { targetId: target.id}
   chrome.debugger.attach(targetArg, "1.3", function(){
     chrome.debugger.sendCommand(
        targetArg,
        "Network.setRequestInterception",
        { patterns: [{ urlPattern: "*", interceptionStage: "Request" }] }
     )
     chrome.debugger.onEvent.addListener(console.log)
   })
})

3. Reload example.com

What is the expected behavior?
Request interception is not supported in the 1.3 version of the DevTools Protocol https://chromedevtools.github.io/devtools-protocol/1-3/Network

Therefore:
- Network.setRequestInterception should fail
- There should be no "Network.requestIntercepted" events being fired

However, everything works fine. (Same behavior when specifying version 1.2.)

What went wrong?
Everything works as it would for the tip-of-tree protocol version and requests are intercepted.

Did this work before? N/A 

Chrome version: 68.0.3440.106  Channel: stable
OS Version: OS X 10.11.6
Flash Version: 

The goal here is to prevent developers unknowingly relying on unstable APIs.

This shouldn't be done retroactively for version 1.0-1.3 as it would break existing extensions.