Michael Kay responded to my StackOverflow post [1]:

XQuery Update 3.0 is only a WG Note - it's unfinished work. The main new
feature relative to 1.0, namely allowing an expression to return both a result
and a pending update list, proved controversial and I think it basically hit a
block where there wasn't enough consensus - or enough effort - to take it
forward.

That leads me to this question:

What version of XQuery Update does Saxon support? XQuery Update 3.0? XQuery
Update 1.0? If the latter, then Saxon's XQuery Update is a superset of XQuery
1.0 which is a superset of XPath 1.0, right?

/Roger

[1]
https://stackoverflow.com/questions/75311657/what-is-the-venn-diagram-for-xpa
th-xquery-and-xquery-update