Esc button on an iPad
There is no escape key on iPad. 

Furthermore, I tried connecting my Apple Wireless keyboard to my iPad via bluetooth. I went  on Google Docs (desktop version) and went fullscreen. The escape key on the keyboard is not recognized. Same with the Smart Keyboard on the iPad Pro line - no physical escape key was made.

We can assume iOS has not support for that kind of key yet and with this long, it’s not in the cards for this platform.

That being said, all sorts of apps have custom on screen keyboards that let you touch to send escape key (like prompt ssh app by Panic). For many apps, the tab key serves the same function as esc but certainly not all.

The actual fix is to engage the vendor of the app (or designer of the web page that assumes an escape key is present) to get them to fix their app/design.