Once upon a time, many x11 developers got threatened of legal actions against implementing XOR cursors in Linux. So no one ever implemented them. But now the involved patents are expired, so there should be no reason to hold back anymore.
While some people have already asked for this feature for accessibility (higher contrast) I’m more interested in this as a feature for better text editing (caret style) mouse cursors. In other proprietary OSs, even when high contrast cursor are disabled, the caret style cursor is still an inverted cursor, that allows it to be 1 pixel wide (as opposed to 3 pixels wide, as it is in many linux desktop themes), making it much easier to select text.
As a matter of fact, a 3 pixel cursor will be literally wider than some text characters, so I’d say 1 pixel cursor should be an high priority feature for any desktop OS.
A demo plugin for this kind of cursor was already implemented and posted in this reddit thread, though it has some glitches (it’s just a demo plugin after all) and will be applied to all cursor instead of just caret style cursors.