What people want from a UI library is to be able to use system components. For example, there should be a way to open the system color picker, to display a system data grid, system date picker, etc. It doesn’t need to be more than that.
You can still use the old ones, but they will look old and may not behave like you would expect (hidpi displays, touch screen, etc).
wxWidgets and its bindings to many languages exists if that is what you want.
Writing an uniform interface to GUI widgets on many platforms is very much a non-trivial undertaking, there are plenty of failed attempts over the years.
Not GTK+, it sticks out like a sore thumb. Qt gets much closer, though in both cases you can tell something feels off.
(e.g., click an active tab in a tab control in Qt and you won't see the dotted selection line like you would in Win32. That normally indicates you can switch between tabs on the keyboard.)
That would look out of place on MacOS or Windows. Serenity has a Win 2000 look.