AnyObject is a special kind of non-protocol constraint. Other non-class types (such as ObjC block types and class metatypes) also satisfy the AnyObject requirement. AnyObject will not every have any requirements, and you cannot write an extension on AnyObject.
Actor is a much more of a normal protocol. It will have requirements when we get custom executors. One can write an extension on it. And beyond the fact that we actor-isolate methods on the Actor protocol by default, it's not really all that different from other protocols for which Swift can introduce or synthesize a conformance. We aren't trying to rename Sendable to AnySendable because structs and enums can implicitly conform to it.
It may even be that we find a reasonable way to allow classes to explicitly conform to Actor in the future, if they satisfy the requirements appropriately. That capability was removed because we're not sure exactly what it should do, not because it's fundamentally impossible to do.
AnyObject is the odd protocol out here. Usually Any means "type-erased", and we should consider that the stronger precedent to follow.
Doug