I hope GTK3 will be still supported for a long time.
You're [not alone][0], so keep hoping!
The GTK 3 ABI is finally a stable target, and it’s still pre-installed in most distributions because lots of built-in apps haven’t upgraded yet. If we can get a sufficient number of binary apps depending on it, distributions will be forced to maintain it and even pre-install it “forever”, the same way Microsoft maintains the Win32 API “forever”.
https://ludocode.com/blog/flatpak-is-not-the-future#the-gtk-problem
[0]: https://ludocode.com/blog/flatpak-is-not-the-future