Which fields can be secured?
Although most attributes can be secured, there are system attributes, such as IDs, timestamps, and record tracking attributes, that can't. Below are a few examples of attributes that can't be enabled for field security.
ownerid, processid, stageid, accountid, contactid
createdby, modifiedby, OwningTeam, OwningUser
createdon, EntityImage_Timestamp, modifiedon, OnHoldTime, overriddencreatedon statecode, statuscode Reference:
https://docs.microsoft.com/en-us/power-platform/admin/field-level-security