• Added automatic directory cleanup in RotatingFileHandler (#2000)
• Added timezone-aware file rotation to RotatingFileHandler (#1982)
• Added support for mongodb/mongodb 2.0+ (#1998)
• Added NoDiscard attribute to TestHandler methods to ensure the result is used (#2013)
• Fixed JsonFormatter crashing if __toString throws while normalizing data (#1968)
• Fixed PHP 8.5 deprecation warnings (#1997, #2009)
• Fixed DeduplicatingHandler collecting duplicate logs if the file cannot be locked (2e97231)
• Fixed GelfMessageFormatter to use integers instead of bool for gelf 1.1 support (#1973)
• Fixed empty stack traces being output anyway (#1979)
• Fixed StreamHandler not reopening the file if the inode changed (#1963)
• Fixed TelegramBotHandler sending empty messages (#1992)
• Fixed file paths in stack traces containing backslashes on windows, always using / now to unify logs (#1980)
• Fixed RotatingFileHandler unlink errors not being suppressed correctly (#1999)

Full Changelog: https://github.com/Seldaek/monolog/compare/3.9.0...3.10.0