The extension is now part of Xlib. It might be disabled through the `XKB_DISABLE=1` environment variable (https://gitlab.freedesktop.org/xorg/lib/libx11/-/blob/master/src/xkb/XKBUse....) but I don't see why we have to support that case.
I think assuming that it is always usable will make the keyboard code simpler, though I have no idea if it exists some environment without it nowadays. All recent desktop environment rely on Xkb to support keyboard layouts, especially when multiple layouts are configured (which are grouped four by four in a single X keyboard mapping, using Xkb groups).
-- v2: winex11: Assume that Xkb extension is availabe.