Under shards persist metadata authoritatively, we've introduced the ability to turn feature flags on kUpgrading (SERVER-101613). This means that those feature flags would turn on in a shard as soon as kStart is sent to that shard. In a scenario where the command is sent to a mixed binary cluster (due to administrator error), it would mean that the feature flag would turn on in some shards while other shards are still an old binary. This ticket aims to investigate the safety of such scenario (i.e. no data loss occur).