FEP-521a has been implemented in #Fedify.
Actors now have the #assertionMethods property, and the #Multikey class has been added. For example, if you look at the the actor from the Fedify Example Blog (https://fedify-blog.deno.dev/users/fedify-example), you can see that it has the assertionMethods property in addition to the publicKey property.
You can try it out in version 0.10.0-dev.196+55cc34d1.