You can in fact hide the home bar indicator during gameplay, however it does require a few steps to setup to do so. The key to hiding the home bar indicator is invoking Guided Access in Accessibility. Once enabled, it is an easy triple-click of the power button after the game loads to hide the indicator as well as prevent unwanted interface elements (Control Center, etc) from invoking by swiping during gameplay. To leave Guided Access, then an easy double-click of the power button and Face ID will bring back the home bar indicator and swiping gestures.
Firstly, open Settings and navigate to General > Accessibility > 'Learning' - Guided Access. Next, Turn on Guided Access and allow Face ID to end Guided Access under 'Passcode Settings'. The parameters are now set up; you can exit Settings.
Now, open the game you wish to play. After the game loads, triple-click the power button and Guided Access preferences will open. Press 'Start' in the upper right of the screen. Now put in a passcode (twice) you will remember (initially only; you will not need to use it, Face ID will handle requests). After the passcode input, the game will run in Guided Access mode without any home bar indicator or interface swiping interventions. Once gameplay has ended, double-click the power button and allow Face ID to end the Guided Access session.
The only thing to remember from this point onward is to launch your game, triple-click power button, 'Start' Guided Access, play game, double-click power button to return to normal operation of iOS. the routine becomes second nature and this appears the most optimal way until Apple (or the software developer) decides to offer a preference for hiding the home bar indicator.