Currently, field security cannot be enabled for the primary fields of an entity.

we can set primary fields read-only or not display them by default.

However, these settings apply to all users.

It is inconvenient to have an exception only for the primary field when we want to control the behavior of the field for each user.

The implementation of the ability to enable field security even for primary fields would allow for more granular per-user control.

When this feature is implemented ,Dynamics 365 Online will more useful.