I haven't found this & might have overlooked it, but is there a checkbox/toggle somewhere to turn the line on the copyright page that says it was created using Pressbooks.com & PrinceXML? The line of code that prints this makes it seem that there should be a toggle somewhere, but I haven't found it & would prefer to not manually edit the plugin files, which requires remembering to do it for any future updates.
For some books I don't mind it being there, but others feel slightly less professional with that line.