ß becomes ẞ. Writing a ß to a ss is a convention that stems from computer systems and typewriters lacking the proper glyph. Although commonly understood, I don't think it is a correct substitution, as argued for the inclusion of the uppercase character to the official alphabet. If the user explicitly wants to keep the dated way of writing, they can explicitly type a double S.Jasper de Waard said:
The question is this: if a type user just typed fußball and makes that word smallcaps, would he/she probably want the ß to be converted to the smallcap eszett, or to (I think more conventional) smallcap ss. And if the latter is the case, how would I make the smallcap eszett accesible?