This seems to be one of the most important symbols editors need for their work.

Is that a general statement, or maybe rather dewiki specific? For the latter I think that in theory, often used characters can be defined (if I understand T95856 correctly).

Well, the need for an en dash may be stronger on dewiki, but in the dewiki special characters menu, I cannot find any dash character at all. And I don't think there's any Wiki that doesn't use any kind of dash. However, I do not know whether the menu varies from Wiki to Wiki, and/or if this is a local configuration issue and/or if there's a thought behind it (e.g. Wikis should use MediaWiki:Visualeditor-quick-access-characters.json because the relevant symbols will differ among Wikis), etc. But at any rate it's important the symbol is made available on dewiki.

Change 205558 had a related patch set uploaded (by Bartosz Dziewoński):
Allow special characters with tooltip or that encapsulate text

https://gerrit.wikimedia.org/r/205558

VisualEditor-MediaWiki is now reusing the list of characters from the wikitext editor, but we were accidentally removing the dashes from it.

It wasn't accidental... these characters (7 of them, I think it was?) would not work as they did in WikiEditor (2010) until a VE core change was made to allow encapsulation rather than replacement.

The dashes would, as they don't encapsulate anything, they only had tooltips defined. I submitted patches to support that already, too (T65227, T70425).

Change 205558 merged by jenkins-bot:
Allow special characters with tooltip or that encapsulate text

https://gerrit.wikimedia.org/r/205558