https://github.com/mongodb/mongoid/blob/dffa8432436f05c611eb857fc014af44401c6b2a/lib/mongoid/copyable.rb#L28

This commit was introduced to fix behavior around extra attributes on documents and cloning (something our team had previously patched for 4.x).

It includes `Attributes::Dynamic` as part of the `clone` method - meaning that the behavior of the class could change totally based on whether or not a document has been cloned yet. As a result it breaks expectations about what should not be allowed on the class, and is probably quite slow as it would force a method cache clear.