Package:              com.codex.composer.api.v1.registry
Canonical Name: com.codex.composer.api.v1.registry.ComposerRegistries & ComposerRegistryKeys

Though in previous versions composer also had a registry for toast serializers, in 4.0 that system (the entirety of toasts) was removed, and that registry went with it, thus the only current registry defined by Composer is a registry of PacketSerializers for overlays.

There's not much to it, it's a registry; it can be accessed via ComposerRegistries and ComposerRegistryKeys:

public class ComposerRegistries {
    public static Registry<PacketSerializer<? extends Overlay>> OVERLAY_SERIALIZERS;
}

public class ComposerRegistryKeys {
    public static final RegistryKey<Registry<PacketSerializer<? extends Overlay>>> OVERLAY_SERIALIZERS_KEY = of("overlay_serializer");

    private static <T> RegistryKey<Registry<T>> of(String name) {...} // Internal
}