Are there any standard apps in iOS that implement isIdleTimerDisabled=true?
Though I haven't found any built in apps that do this, I guess it can be done in Safari using a glitch/hack.

This demo page utilizes the hack, and clicking the button on the page keeps the page awake forever.

https://richtr.github.io/NoSleep.js/example/

It is open source, and works on all iOs versions except for 13.4, but will probably be fixed for that too as well!

Note that it may not work as well as a native app though, as I find that if my power is low and I'm not plugged in, this doesn't work (display turns off)