A report from the documentation maintainer

The codepoint for the uppercase “ß” was added to Unicode as a sort of precaution and to make life easier for people implementing case conversion routines. In Germany, in spite of this, uppercase “ß” isn't actually being used in practice, and in fact there is no official agreement or rule as to what the glyph should even look like (although people keep throwing around “ẞ” as if that was some sort of gospel). It would be reasonable to render U+1E9E as “SS” except for the ambiguity with “in Maßen/Massen”, or as “ß” (i.e., exactly like the lowercase “ß”) except for the bad typography of having a lowercase letter in the middle of a bunch of uppercase ones.

Generally, according to the new German orthography rules we're now supposed to write “ß” after a long vowel (or diphthong) and “ss” after a short vowel. This is not the worst part of the orthography reform but the uppercase-“ß” issue remains largely unresolved.