Field specialization can be applied to both legacy and new code
As we work with our customers on their ever-evolving code bases, we find that new features and components added to these code bases often bring more specialization opportunities.
Our own experience on PostgreSQL supports this observation.
As an example, PostgreSQL 9.6 introduced more-native support for the JSON datatype, specifically, a new indexing scheme and the associated logic. Hotspot analysis on workloads utilizing this datatype revealed new specialization opportunities within that code component.