The GNOME 3.20 beta milestone also marks the 3.20 freezes for the API, ABI, UI, and feature addition. Also, all new APIs must be fully documented and the process of writing the release notes has begun.
GNOME 3.19.90 core changes include many bug fixes throughout the stack, Eye of GNOME now has better image quality when downscaling, many Epiphany browser updates, the GNOME Control Center has a redesigned printer jobs dialog, GNOME Software now supports upgrading from one OS release to another, end-user application review functionality, support for XDG-App and Limba bundles, and various translation updates.
GNOME 3.19.90 application changes include a big overhaul to the GNOME Builder IDE and various bug-fixes throughout.
Arguably the biggest change with GNOME 3.19.90 is the GNOME Software support for upgrading the entire operating system to another release, which will be made use of in Fedora 24 for supporting the graphical system upgrades.
The GNOME 3.19.90 release announcement can be read via the devel-announce-list.