OXPS (Open XML Paper Specification) was Microsoft's competitor to PDF. It never really gained traction among users, who preferred the PDF format. XPS is still around, but most people use PDF instead. With the April 2018 Update, Microsoft ended support for
the XPS format and removed the XPS Viewer app.
If you like, you can reinstall the XPS Viewer. Just follow the instructions here:
How to access XPS Viewer in the Windows 10 April 2018 Update
However, you should know that most third-party PDF readers, if not all, will open XPS files. The same third-party PDF readers include a print-to-file PDF printer, so you can save files directly to the PDF format. Going forward I believe that's the wiser
choice.
I use Foxit Reader, which opens XPS files and saves to the PDF format, but there are plenty of other equally capable (and equally free) PDF readers.