The phrase "classic desktop" refers to the traditional layout and design of a computer screen, similar to how it looked in the past. It is used to distinguish it from newer or more modern designs and interfaces. Full definition
This allows classic desktop apps to be packaged as modern ones for an easy install and removal process. (neowin.net)
In a nod to fans of classic desktop interfaces, the new Fedora includes a MATE variant and classic mode for GNOME. (h-online.com)
If you want to take malware protection one step further, you can enable a feature on Windows 10 to block users from installing classic desktop (win32) programs outside of the Microsoft Store. (windowscentral.com)