No, this will not be a 100%-case guarantee. ABI stability (and lazily bridged NSString on Darwin platforms) dictates that String will always need to handle non-NFC-normalized contents. String has a performance flag that can be set when the contents are known to be NFC. If the bit is not set, the standard library has to treat the contents as non-NFC. ABI stability means that all future versions of the standard library have to handle strings with and without that bit set.
The performance goal is to find more ways to set that bit when we want performant comparison, but your example code won't break. There will always be ways of creating a String with a "leave my bits alone!" option. Whether that is the default, an explicit option, new or repurposed initializers, or it varies by use case, it's totally open to future change.
edit: grammar