New features of Maven 3.9.x:

• Shared local repository (using advisory file locking, Hazelcast or Redis)
• Switchable and alternative transports (resolver "native" along with default Wagon) with default switched to native.
• Chained local repository (for IT isolation between "outer" and "inner" builds)
• Recording reverse dependency tree tracking information
• Pluggable checksum algorithms API (is not tied to MessageDigest anymore)
• Choice of resolver collectors BF along existing DF (parallel POM downloads with BF)
• Split local repository, plus "workspace" support for branched development
• Remote repository filtering (see https://github.com/cstamas/rrf-demo)
• Trusted checksum sources (ability to provide some or all artifact checksums ahead of time)
• Pluggable artifact resolver post-processor, with "trustedChecksums" implementation (that verifies artifacts on resolve)L makes possible to use "untrusted" local and remote repositories and to end up with "trusted" artifacts.
• Help with projects maintenance - warnings about deprecated plugins, goals, deprecated parameters, no existing parameters, read only parameters 
• Add support for "mvn pluginPrefix:version:goal"
• Profile activation by packaging
• Fixing Maven finally to be able to use m-install-p/m-deploy-p 3.x
• JDK 8 as minimum
• ??? add the rest, these were just from top of my head