From the design: 

We will add a ‘featureFlagTopNAccumulator’ feature flag as part of this project to ensure that the new accumulators and expressions don’t get used when the FCV is less than 5.1. This feature flag can then be used with the appropriate ‘register’ macros to ensure that the new accumulators, window functions, and expressions are FCV gated properly.

Additionally, these new accumulators will not be part of the stable API initially, so we will need to extend the REGISTER_ACCUMULATOR_WITH_MIN_VERSION macro to perform an api version check, similar to the ones which exist for expressions and pipeline stages.