You are right. It should be OK to use let here.
I wasn't very sure about this so I asked ChatGPT and I got:
You're right that the
defaultValuein thePreferenceKeyprotocol is declared as avar, and changing it to aletwould indeed break the protocol conformance. The warning about concurrency is becausestaticproperties in Swift are shared across all threads, and mutable shared state can lead to race conditions.