How to disable HTTP/2 in chrome or chromium?
For what it is worth, the flag works.
The issue is that you need to quit EVERYTHING Chrome for it to take effect. Including plugin shims and other chrome tabs and so on.
It is not enough just to add the command line switch.
What happens when you try doing the same thing in WebPageTest (select Chrome as the test agent and add the command line switch in the Chrome tab under advanced settings)
Here's a test I did for my personal site just now and the flag appears to work OK (if you look at the response headers you'll see HTTP/1.1)
An easier way to achieve something broadly equivalent is to use an HTTP Proxy, like https://www.telerik.com/fiddler. This adds negligible additional time to your requests, and (as far as I know) doesn't support http/2 at all (yet); even if it did, I'm pretty sure it would be much easier/practical to switch behavior in than restarting all your Chrome windows.
The advantage of this approach is that it takes effect immediately - disabling and reenabling HTTP/2 becomes as easy as starting and stopping the proxy, without messing with the (if you're anything like me) dozens of Chrome tabs you have open, to StackOverflow and elsewhere :)