There have been vulnerabilities found in the code handling mailslots, but the protocol itself is just a mechanism to do broadcast-based communication. It's old and crufty, dating back to the DOS LanManager days, but I'm sure there are applications out there that still rely on the functionality and, as is typical for Microsoft, the API still exists in modern Windows versions. (The NetBIOS "Browse List" functionality that powered "Network Neighborhood" uses this protocol, for example.)