They basically stopped doing the previously-usual updates for a 7 years to focus on porting to GTK+ 3; GIMP was very tied to GTK+ 2. There were a bunch of much-anticipated features (esp. non-destructive editing) that were finished but couldn't be released because it was half-way through the big GTK transition.

Hopefully the now-impending port to GTK 4 will be a lot smoother and won't be such a disruption to their ability to ship the features they've been working on.