The capital eszett is used rarely, though with surprisingly increasing frequency. This article features some recent uses, including the corporate designs of RWE (a major German electricity company), the SPD (a major German political party) and the University of Weimar.
Though using the capital eszett is wrong according to the official orthographical rules¹, I do not consider its usage a flaw, since a well-designed capital eszett does not inhibit legibility (and at the end of the day, legibility is why we bother with orthography). The latter is because all caps and the eszett both occur with relatively low frequency, let alone together and according to the rules – i.e., ß being replaced by SS in all caps. Also, there was a time where the Duden (who then was the official spelling authority) clearly stated that the using SS instead of ß in all caps should only be a temporary solution until an acceptable capital eszett was found:
»Die Verwendung zweier Buchstaben für einen Laut ist nur ein Notbehelf, der
aufhören muß, sobald ein geeigneter Druckbuchstabe für das große ß geschaffen ist.« Duden, 1919
Moreover, the Rat für Rechtschreibung, the committee that issues the spelling rules, stated in its report from 2010:
[Der Rat] wird aber den Bereich [ß-Schreibung] weiter beobachten, nicht zuletzt dahingehend, ob der Großbuchstabe für <ß> nach seiner Kodierung in ISO-10646 und Unicode 5.1 eine grafische Umsetzung erfährt und sich im Schreibgebrauch etablieren kann, was dann im Regelwerk zu berücksichtigen wäre.
With other words: The official German spelling authority states that it would recognise the capital eszett in its rules, if:
- it gets realised graphically (which already happened)
- it gets established in use (which can only happen, if users actively violate the spelling rules)
Therefore the usage of the capital eszett in order to establish it can be said to be encouraged by the official German spelling authority in some sense.
So, if you use all caps or small caps, I would recommend:
- If it is not a logo or similar: Check, if you really want to use all caps, since they decrease legibility, make the text optically disharmonious and a lot of people feel shouted at by them. Consider using small caps or another type of typographical emphasis (bold, italics, …) instead.
- If you do not have to adhere to the spelling rules for their own sake (e.g., in exams) and you can assure that your text is rendered in a font with a proper capital eszett, use it (GOETHESTRAẞE). This is arguably the alternative which least readers will stumble over. I personally used a small-caps eszett in a situation with several readers who where supposed to spot mistakes, and nobody noticed. The design of a capital eszett is good, if it cannot be read as a B and if it does not stand out amongst the other capital letters of the font, e.g., by being to narrow (see also here).
- Otherwise use SS (GOETHESTRASSE). Never use a small eszett (GOETHESTRAßE), except for forms where names need to be identified correctly and similar.
¹ This does not necessarily hold for small caps, depending on how you interpret the spelling rules.