To satisfy the requirements of the Sendable protocol, a class must [...] have no superclass or have NSObject as the superclass.
Classes that don’t meet the requirements above can be marked as @unchecked Sendable, disabling compile-time correctness checks, after you manually verify that they satisfy the Sendable protocol’s semantic requirements.
For the sake of posterity, I just want to re-emphasize this part of the passage that @aley quoted:
@unchecked Sendable should not be used as an easy 'escape hatch' for satisfying the compiler. You should be very sure that the base class does not violate Sendable semantics before doing this (and the same for your subclass, since you'll lose compiler checking on any new functionality you introduce by using @unchecked).