If the Entity expects uid and and user is not exported - then the default content import does not work.

see 🐛 User 1 isn't the super user anymore in Drupal >= 10.3 Active .

This is still valid after 🐛 User 1 isn't the super user anymore in Drupal >= 10.3 Active . Although it needs to use $this->getAdministator() to get the correct uid to insert.

The issue was, and still is, that if the UUID of the required owner field of an exported entity is not in the importing site the the value of the owner field is discarded, the required field then has NULL - hence failing to import.

I use "owner field" here. It's usually uid but I'm not sure it has to be does it? It is however maybe the most common case of a required field referencing a uuid that might not exist in the system that can reasonably be reassigned to another (default admin user) reference.