Fixed that for you: _American_ writers have always used the em dash. In British English orthography, space-en dash-space is much more common.
> Before the sacred, people lose all sense of power and all confidence; they occupy a powerless and humble attitude toward it. And yet no thing is sacred of itself, but by my declaring it sacred, by my declaration, my judgment, my bending the knee; in short, by my — conscience.
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In any case, the "en-dash", as you seem to suggest, is not equivalent to the "em-dash", but typically used to express ranges or contrast between two words, i.e. "1990–1992" or "push–pull configuration".
In Robert Bringhurst's The Elements of Typographic style – pretty much a bible amongst typographers – he states:
We should “[u]se spaced en dashes – rather than close-set em dashes or spaced hyphens – to set off phrases.” Bringhurst then adds this devastating indictment:
The em dash is the nineteenth-century standard, still prescribed in many editorial style books, but the em dash is too long for use with the best text faces. Like the oversized space between sentences, it belongs to the padded and corseted aesthetic of Victorian typography.
Edit: I dug out the original text of your translated phrase to see if it was Stirner’s or the translator’s use of em-dashes, and it looks like it was direct from Stirner: https://archive.org/details/bub_gb_sQ5RAAAAcAAJ_2/page/n89/m...
> Fixed that for you: _American_ writers