I remember when the debate about adding ẞ was ongoing here on this
list. There were lots of old fonts shown which had a distinct uppercase
ß. I remember that some insisted there was no such letter, pointing to
the pronouncements of the then-current Große Duden, which said that ß
capitalizes to SS, and yet the cover of *that very book*, in all-capital
letters an inch high, clearly showed its title as DER GROẞE DUDEN.
We can find the discussion in the list archives somewhere. Suffice to
say, ẞ does seem to be a real thing and seems to have been so even
before it was recognized.
~mark