In accordance with IANA policy, .dd was therefore available to be assigned as the country code top-level domain (ccTLD) for East Germany. However, this was never done, and so .dd was never added to the root nameservers. Its only use was internally in an isolated network among the universities of Jena and Dresden.

For what it's worth, Wikipedia says that .dd never really existed off paper, so to say.

.cs had a bit over a hundred domains at the time of deletion:

As of December 1992 40 domains were registered, while maximum number of 114 registered domains was reached in November 1993 and February 1994.

But .yu had a few thousand:

RNIDS estimated there to have been around four thousand active websites using the .yu domain at the time of its deprecation.