When we started development towards GTK+ 4, we laid out a plan that said GTK+ 3.22 would be the final, stable branch of GTK+ 3. And we've stuck to this for a while.
I has served us reasonably well - GTK+ 3 stopped changing in drastic ways, which was well-received, and we are finally seeing applications moving from GTK+ 2.
But, GTK+ 4 is taking its time to mature (more on that in another post), and some nice new features (such as font variation support, or Emoji completion) languish unused in master. We also get requests for critical APIs from some of the ported applications.
Therefore, we have decided that it is better to change course and allow a limited amount of new features and API in GTK+ 3.x, by doing a GTK+ 3.24 release in September.
I'm not even remotely versed enough in the world of GTK+ to say anything meaningful about this, but it does seem like a welcome move for developers and users of GTK+ alike.