Maybe a hint?

Maybe a hint?

What I had in mind is that big projects like Linux Mint, MATE, Ardour, GIMP and others would coordinate some efforts to ease GTK usage outside what GTK maintainers are able to provide.

I don't know that those projects have the resources to do what you're suggesting. For example, GIMP is still in the process of fully transitioning from their old libraries to GEGL. They don't have spare developers to devote to maintaining old GTK; if they had more resources they'd want to devote them to speeding up development of their graphics stack rather than maintaining underlying libraries. I agree that maintaining those old libraries would be a worthwhile task, but I don't think projects that are already operating on a shoestring are the place to find the resources.