What's the equivalent of --safe-updates of mysql in MS SQL Server?
There may or may not be such a thing in MS SQL, but why would you want this?

There is already a way to limit what UPDATE affects (the WHERE clause).
There is already a mechanism to safeguard your data when making changes (Transactions). 

Combining the two, we get the general-case solution: "Do your updates in a transaction, with appropriate WHERE clauses to limit what they touch, and make sure the results look right before you commit":

&gt; BEGIN;
&gt; UPDATE mytable SET foostring='NewValue' WHERE id &gt; 16 AND id &lt; 32;
15 Rows Updated

&gt; SELECT * FROM mytable;

  [Omitted -- Make sure it looks right]

&gt; COMMIT;  --- Or ROLLBACK if the SELECT doesn't look right