The most recent Saxon version is 9.8 and we could not migrate because until recently Home Edition did not allow to run compatible mode which prevented to run XSLT 1.0 transformations.
To be frank XQuery/XSLT 3.1 is not at top of our priorities, there is such a tiny percentage of Stylus Studio's users running these in production.
Having said that we are planning to upgrade to the latest Saxon in the upcoming Stylus Studio update.