Symfony 5.4.40 has just been released.
Here is the list of the most important changes since 5.4.39:

bug #57275 Fix autoload configs to avoid warnings when building optimized autoloaders (@Seldaek)
bug #54572 [Mailer] Fix sendmail transport failure handling and interactive mode (@bobvandevijver)
bug #57228 [Mime] fix PHP 7 compatibility (@xabbuh)
bug #57065 [Mime] Fixed MimeMessage::ensureValidity() when a required header is set, but has an empty body (@rhertogh)
bug #57109 [Notifier] keep boolean options when their value is false (@xabbuh)
bug #54694 [PropertyInfo] Update DoctrineExtractor for new DBAL 4 BIGINT type (@llupa)
bug #54913 [Serializer] Fix CurrentType for missing property (@ElisDN)
bug #54797 [PhpUnitBridge] Fix DeprecationErrorHandler with PhpUnit 10 (@HypeMC)
bug #54878 [Filesystem] Fix dumpFile stat failed error hitting custom handler (@acoulton)
bug #54924 [Validator] IBAN Check digits should always between 2 and 98 (@karstennilsen)
bug #54919 [ErrorHandler] Do not call xdebug_get_function_stack() with xdebug >= 3.0 when not in develop mode (@fmata)
bug #54910 [HttpFoundation] filter out empty HTTP header parts (@xabbuh)
bug #54888 [String] Fix folded in compat mode (@smnandre)
bug #54860 [HttpClient] Revert fixing curl default options (@alexandre-daubois)
bug #54839 Fix exception thrown during LDAP_MODIFY_BATCH_REMOVE_ALL batch operations (@phasdev)
bug #54834 [Validator] Check Locale class existence before using it (@alexandre-daubois)
bug #54830 [HttpClient] Fix cURL default options for PHP 8.4 (@alexandre-daubois)
bug #54828 [Serializer] Fix GetSetMethodNormalizer not working with setters with optional args (@HypeMC)
bug #54816 [Cache] Fix support for predis/predis:^2.0 (@mfettig)
bug #54804 [Serializer] separate the property info and write info extractors (@xabbuh)
bug #54800 [WebProfilerBundle] fix compatibility with Twig 3.10 (@xabbuh)
bug #54794 [Strings][EnglishInflector] Fix incorrect pluralisation of ‚Album‘ (@timporter)
bug #54714 [Serializer] convert empty CSV header names into numeric keys (@xabbuh)
bug #54775 [Messenger] accept AbstractAsset instances when filtering schemas (@xabbuh)
bug #54759 [Filesystem] better distinguish URL schemes and Windows drive letters (@xabbuh)
bug #54791 [FrameworkBundle] move wiring of the property info extractor to the ObjectNormalizer (@xabbuh)
bug #54760 [Validator] handle union and intersection types for cascaded validations (@xabbuh)
bug #54776 [Cache] fix: remove unwanted cast to int (Arend Hummeling)
bug #54700 [Dotenv] show overridden vars too when running debug:dotenv (@HMRDevil)

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