*   Raise FrozenError when trying to write attributes that aren't backed by the database on an object that is frozen:

        class Animal
          include ActiveModel::Attributes  
          attribute :age 
        end
        
        animal = Animal.new
        animal.freeze 
        animal.age = 25 # => FrozenError, "can't modify a frozen Animal"
          
*   Add *_previously_was attribute methods when dirty tracking. Example:

        pirate.update(catchphrase: "Ahoy!")
        pirate.previous_changes["catchphrase"] # => ["Thar She Blows!", "Ahoy!"]
        pirate.catchphrase_previously_was # => "Thar She Blows!"

    *DHH*

Please check [6-0-stable](https://github.com/rails/rails/blob/6-0-stable/activemodel/CHANGELOG.md) for previous changes.