Just learned a lesson the hard way.
If you delete a country from shipping page, then re-add it, the tax rate for that country will be reset to 0.
This is because if you delete a country from shipping, it deletes the tax rate. Tax rates are dependent on shipping to the country.
This is a pretty dangerous workflow and would strongly suggest that Shopify include a warning 'When you delete this country, all associated tax rates will also be deleted'.
Nick here from Shopify.
Thanks for writing this post and giving this feedback. I can see how this could be irritating should the tax for any country be removed for any reason. I am going to give this feedback back to our team for future consideration. Thank you again for bringing this up and letting us know.
All the best, Nick