How about looking in Books Online?
allow updates Option
This option is still present in the sp_configure stored procedure, although its functionality is unavailable in SQL Server. The setting has no effect. Starting with SQL Server 2005, direct updates to the system tables are not supported.
This feature will be removed in a future version of Microsoft SQL Server. Do not use this feature in new development work, and modify applications that currently use this feature as soon as possible.
Changing the allow updates option will cause the RECONFIGURE statement to fail. Changes to the allow updates option should be removed from all scripts.
It used to allow direct changes to the system tables (like sysobjects) which was a very bad idea usually.
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