Parsoid can't implement deserializers without a cyclic dependency on core, as the necessary JsonUnserializable interface is internal to core. It would also like to be able to use a consistent JSON codec to implement output in parser tests, in standalone mode, etc.
Other library code (see https://www.mediawiki.org/wiki/Parsoid/OutputTransform/HtmlHolder for instance) would also benefit from being able to use a consistent JSON serializer that supports deserialization to objects.
Desired features are: (a) highly performant, since it is on the critical path of cache hits for article read views, and (b) service-friendly, in case deserialization needs to consult a MediaWiki service, (c) compatible with existing serialization formats to allow transition of ParserCache contents.
A proposed implementation is in https://github.com/cscott/json-codec