This is a recent development and that was designed to support GNOME/RedHat's flatpack , we should not get tricked to believe GNOME would have done this for helping someone else then themselves(do they support this in GTK2 apps too?, I really hate having different file choser with different sorting order of files and folders)
It probably cannot be done automatically in GTK2 apps because of API differences. GTK2 apps could call the portal API manually or put the functionality in another library.
>we should not get tricked to believe GNOME would have done this for helping someone else then themselves
Well they actually did, the flatpak API was designed to support multiple toolkits.
