XML Schema Definitions for all ISO20022 messages are public: https://www.iso20022.org/iso-20022-message-definitions
There are tools that can turn the schema files into POJO/dataclasses/structs/etc in your language of choice, sometimes with proper data validation. Not sure about Typescript, but Java/Python/Golang definitely have those.
It may very well be lost knowledge in certain ecosystems, but generating a valid XML based on the given schema is generally a solved problem. Not sure if the projects adds anything beyond that.