Magento 2.2.5 - Admin Panel has 503 Service Unavailable page under Content=>Pages. I updated this page but the changes are not reflected. Is Magento using this page or not?
Magento's (admittedly lackluster) documentation suggests just intercepting the 503 at the server level and serving your own 503/maintenance page. https://devdocs.magento.com/guides/v2.2/comp-mgr/trouble/cman/maint-mode.html
I think common wisdom among Magento developers though, is to simply edit that file you already found:
pub/errors/default/503.phml. When you enable maintenance mode, that is the file the Magento will load.