I've been out of the gtk loop for a while now—I built a couple apps on gtk2 back in the day. What is wrong with gtk3? Why would this be controversial at all?
I don't have a definitive answer, but I think it'd be from association with Gnome 3, which draws out all sorts of pointless and tiresome flame wars amongst people who thrive on creating (and then wading into) emotionally charged drama about software, of all things. :(
Gtk+ 3 is part of the modern degenerate conspiracy along with systemd and other abominations such as kdbus to destroy Linux by straying from the time-honored ancestral traditions of UNIX which were devised when men were still true hardworking men, that spent hours of sweat and tears configuring their window managers and compiling kernel modules as was ordained by the ancients.
ggs ... if you feel this way then create a for profit service that makes money ... like nginx ... keeping things alive is about having minions to work for you