According to Unicode ß gets converted to SS in uppercase. This is by definition and doesn't change (stability policies, as far as I recall). Even in German you'

According to Unicode ß gets converted to SS in uppercase. This is by definition and doesn't change (stability policies, as far as I recall). Even in German you'll never see ß capitalised as SZ (except when I do it, but I'm a very, very small minority – and now I'm more likely to use ẞ).