Since 2024, the preferred upper-casing of ß in the German locale is ẞ, not SS. See https://en.wikipedia.org/wiki/%C3%9F reference [5] (only in German; https://www.rechtschreibrat.com/DOX/RfdR_Amtliches-Regelwerk_2024.pdf), specifically ibid § 25 E3 on p. 48.

There's a Unicode proposal about this: https://www.unicode.org/L2/L2025/25223-proposal_uc_sharp_s.pdf as well as discussion of this paper (and the change at large) here: https://www.unicode.org/L2/L2025/25228-utc185-properties-recs.pdf (search for L2/25-223).

Qt currently implements the old semantics, by following Unicode, but my software needs to follow the official German rules, which call for the use of ẞ.

I don't know whether there's precedent for implementing rules that are not in Unicode, yet, but having at least some API to implement this at the user level, without having to do it myself would be good.

Work-around: before calling s.toUpper(), do s.replace(u'ß', u'ẞ').