From design doc: 

If the feature flag is on but the extension is not loaded, legacy $vectorSearch will be used. So, what if the router does try to use extension $vectorSearch and annotate the agg request with the IFR flag true, but then the shard doesn’t have the extension loaded and silently uses legacy $vectorSearch?

We decided to handle these scenarios by tassert’ing... For an unloaded extension, we won’t use a named error code for the tassert.