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.
edit: I realized that you want the menu entries to be rendered in the fonts themselves
Regarding Slint and all the other new GUI toolkits, one thing I don't understand is why they all seem to completely ignore any controls more complicated than the basics (buttons, checkboxes, drop downs, spin controls, sliders, radio buttons, and a basic list control).
A To-Do app is a proof of concept, not an endgame.
Beyond those basics I need combo boxes, date time pickers, color pickers, font pickers (fonts are hard, I get it, but my users expect access to system fonts), dockable windows, splits, tabs, modal dialogs, tooltips, masked text input, auto completion text input, context menus, calendar controls, file and directory pickers.
Some of the new toolkits offer an assortment of these, including Slint but they rarely get all of them but the biggest thing missing from pretty much all of them are sophisticated data grids (both list and tree varieties). They need support for custom cell rendering, custom cell edit controls, and variable row heights. They also need to be fast even with tens of thousands of rows.
Oh, and think about your printing story. You don't print a UI obviously but if I'm rendering diagrams and stuff using your rendering facilities I at least need a way to hook them up to the platform's system without just duplicating all of the drawing to a completely different rendering API to make printing work.
This isn't a dig on Slint. I'm eagerly watching it, along with egui, as I desperately look for a cross platform alternative to all the legacy stuff I'm neck deep in. I just can't consider anything that has no good support for data dense applications.