It cannot be unconditionally Sendable right now because of two reasons. First, you are passing the state into the init of ManagedCriticialState. You could be passing a reference type in here which you might be accessing outside of the withCriticalRegion closure. The first problem could be solved with future language extension where the init is taking the State parameter; however, even then the State might contain a reference type which you could escape the reference out through the withCriticalRegion method. So the safest approach here really is to only make this Sendable if the State itself is Sendable.
There is a similar type in NIO that expresses the same constraints: swift-nio/NIOLockedValueBox.swift at main · apple/swift-nio · GitHub