I don't think that Bitmain has a backdoor, I know they do. While it doesn't damage equipment, it does prevent them from mining. This backdoor is in almost all S9s (probably only the first few batches were safe). While you can mitigate it with a hosts file change, I doubt many miners know of this.
It's called AntBleed.Thank you for bringing it up. It's been a long time since I read article about AntBleed. Hardly anyone mentions it these days. Now, the question is, when are they going to use it? If they wanted to damage Bitcoin in favor of Bitcoin Cash dominance then it would be disastrous for the whole crypto community. Bitcoin is one of the most popular cryptocurrencies. Imagine how satisfied some people would be if it happened. The media would quickly spread FUD. I am looking forward to seeing some other company trying to compete over with Bitmain.