Content was removed 

AbstractTypolinkBuilder::build() removed 

TYPO3 v15 removed the method AbstractTypolinkBuilder::build() and the property $contentObjectRenderer. A link builder implements TypolinkBuilderInterface and its method buildLink(). It gets the content object renderer from the request attribute currentContentObject. See Render the custom link format in the frontend.

ext_emconf.php no longer evaluated 

ext_emconf.php files in extensions are no longer evaluated for package metadata. In TYPO3 classic (non-Composer) mode the composer.json must declare the extension version and the providesPackages definition in the extra/typo3/cms section. These two fields are now mandatory, also for extensions that no longer ship an ext_emconf.php, otherwise loading the extension throws an exception.

DatabaseWriter->getLogTable() and DatabaseWriter->setLogTable() where removed 

The DatabaseWriter can only be used with the sys_log table starting with TYPO3 v15.

Page module section markup events removed 

The events BeforeSectionMarkupGeneratedEvent and AfterSectionMarkupGeneratedEvent were removed with TYPO3 v15. There is no direct replacement so listeners that decorated backend layout columns with these events should be removed.

CacheAction "href" key removed 

AddJavaScriptModulesEvent renamed to AddUserSettingsJavaScriptModulesEvent 

The event \TYPO3\CMS\Backend\Event\AddUserSettingsJavaScriptModulesEvent was moved from \TYPO3\CMS\Setup\Event\AddJavaScriptModulesEvent as the system extension setup was integrated into typo3/cms-backend .

ModifyLanguagePackRemoteBaseUrlEvent and ModifyLanguagePacksEvent moved to EXT:core 

The events ModifyLanguagePackRemoteBaseUrlEvent and ModifyLanguagePacksEvent have been moved to typo3/cms-core .

typo3/cms-install has no events anymore as it was made optional.

Extbase documentation rewrite 

The old Extbase documentation chapter has been removed and rewritten. Most pages have a direct successor under the same URL. The list of example extensions has been retired — see the rewritten Extbase documentation for current guidance.