Radeon Pro W6000 series MPX modules are now available to order for existing Mac Pro machines. Unsurprisingly, the proprietary GPUs are expensive and are incompatible with some existing Mac Pro configurations.
https://www.notebookcheck.net/Apple-starts-selling-Radeon-Pro-W6000-series-MPX-modules-and-confirms-compatibility-with-existing-Mac-Pro-machines.554369.0.html