No changes are currently planned for the Adwaita shipped by GTK3. All the theming changes are bound to libadwaita.
Are GTK+3 apps going to stay look old forever?
“Forever” is a long time. As I said: there are no plans for implementing the changes of libadwaita in GTK3. GTK3 is feature and API frozen—and that includes the CSS selectors. The changes in libadwaita require new widgets and new CSS selectors for those widgets.
Additionally, libadwaita is targeted at GNOME.
If you want to use the new style for your applications, you’ll have to port them to GTK4 and to libadwaita.
Also, that specific change is a pretty big behavior break (which is why it needed a blog post in the first place) - and forcing that on existing apps is not exactly a good idea.