I'm living in a German-speaking country where ß is not used at all. Therefore, I do not use ß when I write German texts. Now I'm asking myself if this is technically a spelling error? Or is the ß more or less just a convention?
There is no single set of spelling rules for the German language (just as, e.g. British and US English have different orthographies). German as used in Germany does include ß in certain words (after long vowels and diphthongs), and this is -- within the official orthography -- mandatory. German as used in Switzerland, however, does not.
An analogy in English (although not a special letter) would be the use of u in words such as neighbo(u)r. US spelling does not have a u, but in British English, this is not merely a convention (except in the sense that orthography rules are a convention).
Eszett is mandatory unless there are technical difficulties to write it, like writing on a non-German keyboard, or using a software or font that just doesn't support the letter. Yes, it is a spelling error, but you can substitute it with ss if you can't use ß for some reason. The same goes for the umlaut letters ä, ö and ü.
Duden rule 160:
- Fehlt das ß auf der Tastatur eines Computers oder einer Schreibmaschine, schreibt man dafür ss.
Please note that people may find it impolite if you don't use ß and the umlauts for no reason.
It is more or less a convention to replace "ss", and "sz" by "ß". For more on this topic see:
Nevertheless there are rules in Germany to define when to write "ss", or "ß". Therefore we have and had to learn these rules (including their many exceptions). Up to now German pupils will get a wrong choice marked as an error in school. Also it will be corrected when editing books, journals, and newspapers.
Still this rules are not true for all German speaking countries. In Switzerland the letter "ß" does not exist at all. All German words are spelt with "ss" in Swiss publications. Putting an "ß" would be a definite error there.
To quote the recommendations of the Rat für deutsche Rechtschreibung (Council for German Orthography):
Steht der Buchstabe ß nicht zur Verfügung, so schreibt man ss. In der Schweiz kann man immer ss schreiben.
If the letter ß is not available, ss is written. In Switzerland, one can always write ss.
If you mix ss (instead of ß) with other other umlauts (instead of ae, oe, ue), people will think you're from Switzerland.
For texts that have no association with Switzerland, it will be seen as a spelling error.
Those alpine rustics and their quaint spelling ways...
When Germany took a cue from the Swiss and sensibly changed its orthography so that ss instead of ß follows a short vowel, the Swiss could have reciprocated by replacing their ss after a long vowel or diphtong with ß.
Maybe a cavalry detachment should be sent into Berne to help them along.
In the meantime you could start a campaign of subversion by sprinkling ß's throughout your texts, hacking the NZZ webserver to change ss to ß, and defacing street signs in nightly raids. But I perfectly understand if you decline my suggestion.
The people calling for abolishment of ß will be defeated. ¡No pasarán! On the contrary, modern computer technology allows me to call for wide-spread use of the lovely capital ß, which has been in Unicode as character "ẞ" U+1E9E LATIN CAPITAL LETTER SHARP S since 2008. And if you are viewing this text set in the Arial typeface on a Windows machine, you just saw it in the preceding sentence.