The setFeatureCompatibilityVersion command waits for writes to occur on a majority of nodes, but not all. It would be useful to be able to set the write concern higher so that users can rely on it not to return until they can safely change their binaries. This should be as easy as setting the supportsWriteConcern boolean to true.

This would simply be an extra wait on the system last OpTime after the command succeeded, and would have no affect on the w: majority writes done as part of the command.