Already possible in tableau desktop but now Web authoring supports the capability to have offline maps on tableau server. Tableau Online didn't have this issue as tableau online is Hosted by Tableau so always has access to map tiles usually delivered over the internet.
We’re making it easier to access and analyze spatial data on Tableau Server with offline map support. You can now create maps using the offline map style in web authoring, ensuring that maps remain accessible to all users—especially in organizations with strict internet access requirements.
