.$d$ is OS/2 Planner data
Key Features: .$D$ File Extension
What is an .$D$ file?
The $d$ file extension is associated with the OS/2 Planner, a tool for OS/2 operating system developed by IBM.
The $d$ file stores some kind of data.
The OS/2 Planner seems to be obsolete product without support from developer.
This $d$ file type entry was marked as obsolete and no longer supported file format.
This type of file is no longer actively used and is most likely obsolete. This is typically the case for system files in old operating systems, file types from long discontinued software, or previous versions of certain file types (like documents, projects etc.) that were replaced in higher versions of their original programs.