currently we are 99.99% certain that the QuarkXPress 2019 XDK (QuarkXPress interface for XTensions) will stay compatible with QuarkXPress 2018.
So that means that most XTension that run in QX2018 will also run in QX2019.
- The XTension makes use of a feature that is fundamentally different in 2019. Tables come to mind.
More likely, the XTension developer hard coded a version check into the XTension. Then they need to recompile the XT, please contact the XT vendor for compatibility information.