I'm planning to use magento completely through the REST API. I have checked https://devdocs.magento.com/guides/v2.0/rest/bk-rest.html however I'm unsure if all of the functionality of magento is exposed through the rest api. My question is if the whole functionality is not exposed are there any publicly available plans for that?
yes magento 2 have almost all functionality which is there in UI version is also available in Rest Api as well ..
Here i am sharing link for Rest Api - https://devdocs.magento.com/swagger/
But in-case if you found some functionality which is not there in Rest API - then yes its publicly open so you can extend/override existing functionality - also you can create your own custom REST Api as well in magento 2 as its scaleable platform so.
Hope it helps