I should add, if you build Geany and plugins yourself you can choose what you want to do.
Similarly distros can choose what they decide to provide. Some are actively trying to move away from GTK2 probably to move toward not installing more than two GTK versions by default when GTK4 is released later this year. Also they may be anticipating that support for GTK2 will drop off when GTK4 is released. But its their choice, Geany itself works with both GTK2 and GTK3 and many of the Geany devs now use the GTK3 version.