That's 'cause by 3rd Ed. they were already starting to phase out all of the previous content, such as Planescape, and Ravenloft - especially Planescape. Because their new Lords and Masters, Wizards of the Coast, were insistent upon pushing Forgotten Realms and Eberron onto players at events and advertising, and kept giving Planescape, and Ravenloft, the shaft on a regular basis. So make of that whatever you will - Planescape was never incompatible with 3rd. Ed. It was just that WotC were intentionally trying to kill it (popular theory is that, like Firefly, it was too awesome for its own good). This isn't the first time of WotC ignoring player interests in favor of pushing what they personally thought was a 'better product'. They also kept ignoring player interest in d20 Modern.
So yeah. Map is still valid, if simply because Planescape never stopped being relevant to the Cosmology of 3rd Ed. Even though WotC went out of their way to try and kill it dead.
