Have you seen libAgar (https://libagar.org)? Cross platform support is certainly there, covering everything from windows XP (and earlier) to *bsd and SGI IRIX. I'm not sure what all having support for accessibility requires as I've never had to worry about it, but am curious if 1, agar has what's needed, and 2, what exactly is required of a GUI library for accessibility. Screen reader support? (Are there SR standards for desktop applications)? Dynamic scaling? High contrast?
(For embedded and/or touch first UI, LVGL is pretty nice, but probably lacking any semblance of accessibility features apart from keyboard navigation, but you could hook that yourself).