Problem/Motivation
Currently we set $entity->original during the saving process however the entity object does not have such a property and therefor on content entities the magic methods __isset, __get and __set will be involved. In order to make the original property official and do not involve the magic methods it makes sense to create a property on the entity object for it.
Proposed resolution
Create a property $original on \Drupal\Core\Entity\Entity.
Remaining tasks
Review & Commit.
User interface changes
None.
API changes
None.
Data model changes
None.