"Now a debatable and desert land" meaning
Given that modern English has largely left these terms, as they were meant in Tolkien's day, by the wayside, it might be worth adding a comment on what is actually meant.
Debatable these days means a topic that can be discussed, or debated. Historically, when it was applied to a location as in this example, it meant territory that was claimed by two countries. The Crimea is the most noteworthy modern example of this, though it usually doesn't go that far. Hans Island is about halfway between Greenland and the Canadian island of Baffin Island; both Canada and Denmark lay claim to it, but neither has gone beyond saying they own it and the occasional bit of posturing.
Desert is a little harder to be certain of, because the modern meaning could well apply - South Gondor could well have suffered the same fate that the Sahara did and become an actual desert. However, another definition that fits is "a wild uninhabited and uncultivated tract" - in other words, a place nobody lives or does farming. Given how undeveloped the science of climate change was when Tolkien was creating these maps, it seems likely that he would not have known that areas can simply become deserts without the influence of supernatural forces. In his mythos, I supect that fate would be reserved for places of great evil, like Mordor and...did Morgoth ever actually claim a whole country like that? I can only find references to fortresses.
In short, I suspect that "now a debatable and desert land" means "Gondor and Umbar [I guess - my Middle Earth geography isn't that good outside of the places visited by the Fellowship] both claim this land, but nobody actually lives here."
184.108.40.206 21:15, October 13, 2016 (UTC)