Question: Can someone please suggest that in a scaled Sitecore 9 environment for some reason if the Roles other than CM and CD are down, how the Websites will be impacted? 

If the xConnect, Marketing Automation, xDb Reference Data Roles are down, will the CM and CD roles will be down as well or we have any configuration to avoid such dependency?

Answer: 
With xConnect off, the website will keep working BUT if you have any xConnect/Tracker code that is not null checked, it will fail. 

`Sitecore.Tracker?.Current?......`

or 

```
if(Tracker.Current?.IsActive == true)
{
    // do stuff
}

```

All of the Analytics related actions in Sitecore forms will cause the forms to stop working. And action that happens after the failing analytics action will not run.

For Marketing Automation, Processing Engine and xConnect Search Indexer are just services and run out of channel from the website. None of the MA will run and any xConnect searching will be out of date.
Well i just tried on an otherwise empty install of 9.1. 

At least the homepage and a basic edit and publish would work. Other stuff like Experience Profile would just say An error has occurred. 

It would be far safer to run an instance of XM1 instead.