Prominent features of PulseAudio 9.0 include support for sample rates up to 384 kHz, the implementation of a memfd-backed shared memory transport, significant improvements to the automatic routing functionality, as well as the adoption of the C11 C standard instead of C99.
Furthermore, it looks like PulseAudio 9.0 comes with LFE (Low-frequency Effects) remixing disabled by default, which was enabled as part of the PulseAudio 7.0 release, the module-role-ducking and module-role-cork modules received various enhancements, and webrtc-audio-processing 0.2 or later is now required.