It's a bug. There is a capital eszett now, as of 2017. https://en.wikipedia.org/wiki/%C3%9F
[1] http://std.dkuug.dk/jtc1/sc2/wg2/docs/n3227.pdf
[2] https://www.unicode.org/policies/stability_policy.html#Case_...
I've recently re-discovered it, though and somehow this time around actually taken a liking to it, and have smuggled it into a few reports I've had to prepare since then.
There are also non-invariant case-modifying examples, where uppercase -> lowercase -> uppercase (or l -> u -> l) does not produce the input string.