It would be useful to have a jsCore passthrough that sets featureCompatibilityVersion=3.4 or even a featureCompatibilityVersion=3.4 variant.

One motivation is that our old update implementation is used when the featureCompatibilityVersion is 3.4, and our new system is used when the featureCompatibilityVersion is 3.6, so there is currently no integration test coverage for our old system. A jsCore passthrough with featureCompatibilityVersion=3.4 would be helpful, but it would also be good to run shard_key_immutable.js with featureCompatibilityVersion=3.4, and that test is not in the core suite.