Breaking: #107943 - Frontend and backend HTTP response compression removed 

See forge#107943

Description 

The TYPO3 frontend and backend application allowed to compress its HTTP responses using the configuration toggles $GLOBALS['TYPO3_CONF_VARS']['FE']['compressionLevel'] and $GLOBALS['TYPO3_CONF_VARS']['BE']['compressionLevel'] .

This feature was always disabled by default and has been removed: TYPO3 will no longer compress its HTTP responses. Response compression should be applied by web servers and not by the application layer. Removing this feature avoids collisions when both apply compression and allows web servers to use improved algorithms like brotli or zStandard if HTTP clients signal compatibility in HTTP requests.

Impact 

TYPO3 can not compress its HTTP responses anymore and hands this task over to the web server. HTTP response compression had to be actively enabled in instances.

Affected installations 

Instances that configured $GLOBALS['TYPO3_CONF_VARS']['FE']['compressionLevel'] or $GLOBALS['TYPO3_CONF_VARS']['BE']['compressionLevel'] to non zero values may be affected and should check whether the web server applies HTTP compression, indicated by a HTTP response header like Content-Encoding: gzip when requesting a frontend and backend document with a HTTP header like Accept-Encoding: gzip, deflate. The default configuration of commonly used web servers enables this feature.

Migration 

The configuration toggles for the backend $GLOBALS['TYPO3_CONF_VARS']['BE']['compressionLevel'] and the frontend $GLOBALS['TYPO3_CONF_VARS']['FE']['compressionLevel'] are obsolete, existing settings in settings.php configuration files are actively removed when first using the install tool after upgrade to TYPO3 v14.