GStreamer 1.18 finally does away with the Autotools build system in favor of Meson, Cerbero is supported as a meta build system for platforms like Windows/Android/macOS/iOS, a new gst-tester test utility, and a wide variety of other core improvements. The complete release notes / concise change-log for GStreamer 1.18 has yet to be published.
On particular improvement crossing the wires this week is work on better high bitrate streaming support with GStreamer RTP . That work will be found in the upcoming GStreamer 1.18 release with better RTP performance.
The brief GStreamer 1.18 RC1 release announcement can be read on gstreamer.freedesktop.org .