These accounts are created by default with SQL server installation. I consider them harmless. I suggest you do not delete them.
The SQL Writer service uses the NT Service\SQLWriter login to connect to SQL Server. Using the NT Service\SQLWriter login allows the SQL Writer process to run at a lower privilege level in an account designated as no login, which limits vulnerability. If the SQL Writer service is disabled, then any utility which in relies on VSS snapshots, such as System Center Data Protection Manager, as well as some other 3rd-party products, would be broken, or worse, at risk of taking backups of databases which were not consistent.
Windows Management Instrumentation (WMI) must be able to connect to the Database Engine. To support this, the per-service SID of the Windows WMI provider (NT SERVICE\winmgmt) is provisioned in the Database Engine.
For NT Service\MSSQLSERVER and NT Service\SQLSERVERAGENT, if they are no longer the service account for SQL server service and agent service, you should be able to disable them , provided that they don't own any objects or the like.
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