Introduced in 4.1, blog post:https://symfony.com/blog/new-in-symfony-4-1-faster-serializer

It is mentioned, but only in "performance" subpart of docs:https://symfony.com/doc/current/components/serializer.html#performance

General serializer docs regarding Normalizers:https://symfony.com/doc/current/components/serializer.html#normalizers

Creating your own Normalizer misses this completely:https://symfony.com/doc/current/serializer/custom_normalizer.html