Oracle 12c upgrade - backwards compatible with 11g?
Usual rules apply. Code changes should not be required but something will break if code is more complex. Also there will be some SQL plans regressions.

Official documentation about deprecated features is there: Deprecated and Desupported Features for Oracle Database 12c

Also you can check Metalink note 567171.1 about _fix_control parameter. This parameter is used to enable or disable certain bugfixes which affect query behaviour. In 11.2.0.4 the view V$SYSTEM_FIX_CONTROL contains 854 rows which means that there are so many potential quirks while upgrading to the newer version of Oracle Database. I am sure that 12c contains much more rows.