[EdSchouten]

That's not entirely true: with CBOR you can add custom data types through custom tags. A central registry of them is here:

https://www.iana.org/assignments/cbor-tags/cbor-tags.xhtml

This is, for example, used by IPLD (https://ipld.io) to express references between objects through native types (https://github.com/ipld/cid-cbor/).