Apparently it does.

https://stackoverflow.com/a/49011455

That’s not something I care about, though. I don’t mean to imply that JSON feeds are objectively superior to RSS and Atom, I just mean to share what my personal observations are, which might be useful to someone else too.

XSLT 3 (via XPath 3.1) has support for maps, arrays, and parsing JSON to/from those or an XML representation. The XML representation is easier to work with in 3.

There's a draft version of XSLT 4 that Saxon supports which adds more capabilities for working with map and array types.

I guess it's like you can't transform XML with JSON-T :)

Seriously, yes, the really old XSLT does not support JSON. But these days it is much easier to find tools that read or write JSON than to find decent XML support.