As of 2017, after a 100-year delay, there is an upper-case
version of ß.
ß was introduced around 1900, and an upper-case version was planned
at that time, but the glyph wasn't designed in time to be widely
adopted. It was left behind, and for more than a century the only
correct upper-case version of ß was SS.
As of 2017, the official German orthography permits an upper-case ß,
which ias been introduced into Unicode as ẞ. Many fonts and
computer input mechanisms now support it. For example, when
composing this article I found that in the default compose-key
bindings on my Linux system, just as «compose s s» generates the
lower-case ß, «compose S S» generates the upper-case ẞ.
Uppercasing ß as SS is still officially permitted.
More details: Ralf Herrmann, “The Capital Sharp S is now part of the