I don't think Elasticsearch ever returns the HTTP status code 206 Partial Content, so I suspect this code came from something else in your environment. "Partial shard fail" isn't a thing, shards can only completely fail.
This option is not set to false on my cluster settings or on queries so I assume that ES could in extreme circumstances respond with a partial answer.
Am I right ? And If I am, how could I replicate this ?
I've tried to restart data nodes multiple times but I wasn't able to reproduce this, even if I restart 2 of the 3 nodes at the same time I get a "hard" failure but not a partial response (which is expected).