For backwards compatibility reasons, the capitalization rules can't be changed for existing characters. So normalizing by naive case-folding now requires at least three steps:

    "ẞ".to_lower() → "ß"
    "ß".to_upper() → "SS"
    "SS".to_lower() → "ss"

(there's a standard for how to compare strings case-insensitively that doesn't involve repeated case-folding, but it's much more complex)