SQL Server supports a proprietary extension to the ANSI standard UPDATE syntax: it allows a FROM clause to be used so that tables can be joined in an UPDATE statement.
Besides being non-standard (and hence impacting cross-platform portability), it also has the risk that improperly coded join criteriia may result in undefined behaviour, without giving an error or even a warning message.
With the introduction of the MERGE statement in SQL Server 2008, there is now an ANSI-standard alternative that does everything UPDATE FROM does (and then some), without being non-standard and without exposing undefined behaviour. As a result, the proprietary UPDATE FROM extension is no longer required.
(Note that the above applies to DELETE FROM as well)