Ummmm what?
SDL is basically timeless as it's just a layer that abstracts various platforms (Wayland, X11, Mac, Windows, game consoles…) to offer one single API for just making a window with a GPU context, receiving input and stuff.
GTK is a really solid toolkit with a strong, consistent UI design philosophy. The upcoming GTK 4 transitions everything to GPU-based rendering without reinventing the whole toolkit from scratch (unlike Qt with QML, Windows with WPF and UWP…). GTK suffers from lack of funding sadly, so it's a slow project, but the current state of GTK4 is already pretty impressive.
Qt is a monster framework with corporate interests working on it, but it's also really solid and QML/QtQuick was basically Flutter years before Flutter (so actual Flutter is just Google NIH syndrome…).
"Simple" UI libraries like LVGL are always bound to make your life not simple. They usually don't support accessibility, lack support for various platforms (you've just had to port that one to Wayland), have various limitations and so on.
Flutter is not even simple.. it needs the whole Dart VM or compiler whatever monstrosity to run.