Occasionally I want to know since when PrinceXML supports a specific CSS selector. But it's harder to find this than it sould be.

How about adding a column to the documentation that states the minimum Prince version that supports a specific selector or feature? For example:

The Relational Pseudo-class

:has( <relative-selector-list> )

The relational pseudo-class represents an element, if either of the relative selectors in the list, when evaluated with the element as the :scope element, match an element.
Supported since: Prince 14.2