Problem/Motivation
After the introduction of Entity UUID we have in some cases substituted Entity ID in favour of this. When using UUID with Views we found that the generic Entity Argument Validator (\Drupal\views\Plugin\views\argument_validator/Entity) support only Entity ID. As we needed support for validation of Entity UUID we thought of adding support for this as well.
Update
Motivation escalated to support all base fields, not only UUID. For the User entity we now support fields like email and username.
Proposed resolution
Instead of load entities only by ID's, we are loading them by property based on the realField defined in the argument. This way the generic validator will be able to handle all argument types.