Aren't browsers implementations of the XSLT and XPATH standards really out of date to start with?
What would be a modern equivalent of a JSON to HTML transformer?
JSON is much easier to directly access and manipulate in JS compared to XML, so I think you'd just fetch() it directly and then plug the data into something like lit-html. Which probably doesn't need bundling either since it's just template functions.
They are (were), and the implementation was poor anyway - the transform is not dynamic, and not even progressive. It's just a high-latency legacy static templating engine you can't upgrade.