@elextr I don't really see this as a valid approach to support both, more of a way of not having the gut of saying GTK2 version is gone (no offense, I just didn't find a nice way to put it). If really the other one is dead, just say that if somebody wants the GTK2 version they oughta use the one from GP vX.YY, which is older but that's life. I feel like having both is meant to lead to duplication of effort, or worse having some people contributing to the GTK2 and some to the GTK3 version.
Anyway, it's just my 2¢ and it's up to the maintainer of the plugin (which, even if I fixed a bunch of big issues I'm not), and for myself I don't really care as I use GTK3 but for testing purposes anyway.