It would be helpful for testing if we had an expression that was registered with a feature flag and enabled in the current FCV. We should have two expressions (one that is fcvGated and one that is not). We could then use these expressions to write stable tests for any feature flag behavior, such as FCV downgrades and our registration macros, such as REGISTER_DOCUMENT_SOURCE_WITH_FEATURE_FLAG. It's difficult to write these tests now because features enabled behind feature flags change for each server version.

I linked a few tickets that would use these expressions in testing.