- BC Warning: Fixed SendGridHandler to use the V3 API as V2 is now shut down, but this requires a new API key (#1952)
- Deprecated Monolog\Test\TestCase in favor of Monolog\Test\MonologTestCase (#1953)
- Added extension point for NativeMailerHandler::mail (#1948)
- Added setHandler method to BufferHandler to modify the nested handler at runtime (#1946)
- Fixed date format in ElasticsearchFormatter to use +00:00 vs +0000 tz identifiers (#1942)
- Fixed GelfMessageFormatter handling numeric context/extra keys (#1932)
Full Changelog: 3.8.1...3.9.0