I wonder how feasible would it be to support a fork that added that feature. It seems self-contained enough to not require any public API changes. Just building the GTK source with some flag would turn on that feature, and dynamically linking against that GTK lib.so would make the thumbnails view accessible to the user.

Did anybody care enough to try?

(I personally don't; I use the file manager for any views I want, and drag the files from it onto an app, or onto a file chooser dialog. That feature, which is also present in macOS, was very much worth reproducing.)

Gtk-mushrooms was one such fork, but I think gnome will make it pretty hard to maintain a fork if one starts getting popular.