in note 1169017.1 oracle announced that the setting of “similar” for the cursor_sharing parameter will be deprecated as of 12c: “The ability to set this will be removed in version 12 of the Oracle Database (the settings of EXACT and FORCE will remain available). Instead, we recommend the use of Adaptive Cursor Sharing in 11g.”
the current documentation is up to date and “similar” is not mentioned anymore, but:
SQL*Plus: Release 12.1.0.1.0 Production on Fri Jun 28 15:33:34 2013 Copyright (c) 1982, 2013, Oracle. All rights reserved. Connected to: Oracle Database 12c Enterprise Edition Release 12.1.0.1.0 - 64bit Production With the Partitioning, OLAP, Advanced Analytics and Real Application Testing options SQL> show parameter sharing NAME TYPE VALUE ------------------------------------ ----------- ------------------------------ cursor_sharing string EXACT SQL> alter system set cursor_sharing='similar'; System altered. SQL> show parameter sharing NAME TYPE VALUE ------------------------------------ ----------- ------------------------------ cursor_sharing string similar SQL>
so, the ability to set this is still there …
Daniel Westermann's Blog 
Hi Daniel,
In the same note Doc ID 1169017.1 if you read through it they say:
“Note: In 11.2.0.3 and 11.2.0.4, although setting the parameter to SIMILAR will still be allowed, it will effectively behave as though cursor_sharing was set to FORCE.”
So looks like SIMILIAR=FORCE from 11.2.0.3 and on. It wasn’t maybe so important to block it if it no longer works that way