Files with d suffix may also potentially be old audio files from from AT&T Bell Laboratories. Most likely obsolete.
D file extension - SPPACK audio
What is d file? How to open d files?
File type specification:
File extension d seems to also have been used for old SPPACK audio format from AT&T Bell Laboratories. Most likely had some kind special use, like saving speech, but it is no longer actively used.
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.