I wouldn't recommend it. XF expects them to be there, and there are scenarios where we'll complain if they don't exist. Internal data also contains some other stuff which isn't offloaded by default.
We also cannot guarantee that any add-ons (those which haven't followed XF standards) haven't written directly into those directories and therefore won't be offloaded.
You should, however, at minimum be able to remove data/avatars, data/attachments, data/video (in XF 2.1), data/resource_icons and data/xfmg. And internal_data/attachments, internal_data/file_check, internal_data/image_cache, internal_data/sitemaps.
We also cannot guarantee that any add-ons (those which haven't followed XF standards) haven't written directly into those directories and therefore won't be offloaded.
You should, however, at minimum be able to remove data/avatars, data/attachments, data/video (in XF 2.1), data/resource_icons and data/xfmg. And internal_data/attachments, internal_data/file_check, internal_data/image_cache, internal_data/sitemaps.