> I worked on projects which refused to use anything more modern than XSLT & XPATH 1.0 because of lack of support in the non Java/Net World (1.0 = tech from 1999).

For some things that may just be down to how uses are specified. For YANG, the spec calls out XPath 1.0 as the form in which constrains (must and when statements) must be expressed.

So one is forced to learn and use XPath 1.0.