Libadwaita 1.0 has been released, just at the end of the year.
Libadwaita is a GTK 4 library implementing the GNOME HIG, complementing GTK. For GTK 3 this role has increasingly been played by Libhandy, and so Libadwaita is a direct Libhandy successor.
Libadwaita is quite controversial, as aside from dark mode and a (promised) colour API, applications that use Libadwaita cannot be themed. It’s all the result of developers being unhappy us pesky users get to decide what our computers look like, so they decided to prevent users from theming their systems at all. GNOME’s own applications will surely transition to it, and it remains to be seen if the wider Gtk developer community will opt for it as well.
Libadwaita hjas already led to two major departures from GNOME, and other Gtk-based desktop environments, such as Cinnamon and Mate, may follow.
libadwaita is like systemd: a nasty, oozy, dark hole that no one deserves to be punished with.