The capital ẞ (U+1E9E ẞ LATIN CAPITAL LETTER SHARP S) was adopted into German orthography in June 2017 (ref: https://en.wikipedia.org/wiki/Capital_%E1%BA%9E). I am no expert in the German language, but the current behavior in Chrome Canary (v63.0.3218.0) does not appear consistent - http://jsbin.com/jilenulubu/1/edit?js,console ß ('U+00DF') → ß ('U+00DF'; toLowerCase) ß ('U+00DF') → SS ('U+0053U+0053'; toUpperCase) ẞ ('U+1E9E') → ß ('U+00DF'; toLowerCase) ẞ ('U+1E9E') → ẞ ('U+1E9E'; toUpperCase) I'm not sure what the expected behavior should be, but I would expect the capital ẞ (U+1E9E) to decompose to "SS", or change the behavior of the small ß (U+00DF) to now return a capital ẞ (U+1E9E) upon conversion to upper case.
Not addressed here, but maybe a different bug report is needed to target CSS transforms? (related: https://bugs.webkit.org/show_bug.cgi?id=31563)
This works correctly according to Unicode 10 (and earlier versions). ftp://ftp.unicode.org/Public/UCD/latest/ucd/SpecialCasing.txt only has special casing for lower case ß, and we just use a Unicode compliant implementation in ICU. This is also addressed at <http://unicode.org/faq/casemap_charprop.html#11>. I don't know if there was any discussion of changing this given the newest orthography changes in June.