The German "ü" – a High-Flyer
With 288,402 occurrences, the umlaut 'ü' is the one used by far most frequently in domains with the German ending .de. Second comes the 'ä', which is used in 210,012 .de domains, followed by 'ö' with 159,123 occurrences. In addition to these three umlauts, there are more than 90 other caracters today, including for instance the 'é' or the 'ç', that you can use in the name of your .de domain.
You find more detailed information about the history of .de domains, i.e. when and how DENIC launched IDNs and which domain was the first to contain an umlaut, at https://www.denic.de/en/whats-new/news/article/idn-start-up-phase-now-completed-at-denic/.
Status: 1 September 2021