We covered the story of B&N dropping NOOK for Mac just a few days ago. It seems you can still access and download the software though. The main page for NOOK mobile apps heavily promotes NOOK for iOS, Windows 8, and Android. But you can still grab NOOK for Mac and NOOK for older Windows on the same page.
Just because these apps are available for download does not mean B&N plans to update them. B&N has made one bad move after another this year. It has certainly sent mixed messages to customers with some of its moves. Will NOOK be updated for OS X Mavericks? That is not certain at this point. The good news is you don’t really need the Mac or old Windows versions to read your B&N books on your computer.