The screen menu is something very old. It came from the DOS days in the 80's. Basically a column along the right-side of the screen with text in it. Each line of that column was a menu item. So you had the command-line and the screen menu, and that was it - unless you also used a tablet.
The nice thing about screen menus was that they'd update in context. E.g. while the line command is active, the relevant screen menu would be displayed for options useful in the line command.
Since around R10 (though I cannot remember exactly which version) the pull-down menus were added to the top of the screen. Then later toolbars, palettes, and finally the ribbon. The screen menu has previously been deprecated as a Legacy item (quite a while back), and recently it's been removed from the CUI interface entirely. It's still inside the CUI(x) file, but just never shown for editing.
Other stuff in the Legacy deprecated branch is Tablet areas, Tablet buttons and Image tiles. So taking the example of what's happened to screen menus, it's possible that these might also "disappear" in the future.
My best advise is to start moving your customizations into some of the newer UI elements. E.g. Ribbon panels - they provide the same functionality as you got from the screen menu (e.g. a ribbon specific to the current command opens when that command is active). You could use some of the others as well, e.g. tool palettes / toolbars / pop-up menus / pull-down menus / etc.