In TYPO3 Version 4.3 a deprecation log has been introduced. It logs the use of outdated functions of the TYPO3 Core. This is a hint for Admins or Developers of Extensions to update their extension. The output file is written to typo3conf/deprecation_....log.
But in some cases this file grows to fast. In this situation you may want to disable this function.
You can search it in the install tool. In "All Configuration" you find "enableDeprecationLog".
There are different settings. An empty field disables.
Another way is to directly edit typo3conf/localconf.php.
Add there the line:
$TYPO3_CONF_VARS['SYS']['enableDeprecationLog']= '';
Afterwards delete the temp_CACHED_... files.