On which version of Magento was varnish caching introduced?
Varnish start from magento 2.0,
You can check varnish related docs file from magento's own documentation list,
http://devdocs.magento.com/guides/v2.0/config-guide/varnish/config-varnish-install.html
They cover magento 2.0,2.1 and 2.2 Version's information.
