It's a bit murky because we didn't realize at first the problems with PDF-XChange Printer 2012 (aka PDFX5) co-existing with PDF-XChange Standard (aka PDFX9). But I would say that starting around 6.5.1709 when PDFX9 was introduced, PDFX5 was no longer available. There were several updates and bug fixes to the PDFX9 interface though, so you want to be on 6.5.1721.0 or higher.
PDF-XChange 3.0 (aka PDFX3) has remained supported all along.