I guess you mean not compatible.
Anyway, yeah, the only officially supported version according to AMD's website is 16.04. If they release a version compatible with 18.04, it'll likely be after it's officially available.
Besides, I think AMD recently announced it would contribute to Mesa (or maybe I'm confusing with the Vulkan part only?). Don't expect a bright future for AMDGPU-PRO, it'll probably be dropped at some point like Catalyst has been.