The correct technical designation for a D-sub connector with nine pins is DE9.

It’s early and eyes are still a little blurry, but I’m not seeing a cite?

Wikipedia fleshes it out a bit:

The D-sub series of connectors was introduced by Cannon in 1952.[3] Cannon's part-numbering system uses D as the prefix for the whole series, followed by one of A, B, C, D, or E denoting the shell size, followed by the number of pins or sockets

No links to a primary source, but seems plausible.

https://www.ittcannon.com/d-sub

https://ittcannon.canto.com/direct/document/h10hf84e3l4n77ck...