Which SQL Server/Windows logins require the sysadmin role?
I'm not sure about NT AUTHORITY\SYSTEM. But according to the SQL Server documentation the others are added to the sysadmin role by SQL Server Setup, so I'm guessing it is required.

https://docs.microsoft.com/en-us/sql/database-engine/configure-windows/configure-windows-service-accounts-and-permissions?view=sql-server-2016#DE_Prov