This article shows you how to turn it off with a workaround you could use instead.
Why Is There a Bar at the Bottom of My iPhone?
Simply put: Because there’s no more home button.

The screen now shows you more information, and new gestures have made handling the iPhone faster.
In fact, it’s only there to remind you to swipe up from the screen’s bottom edge.
Notice aSwipe up to unlockmessage appears above the bar when you wait for a moment.

Can You Get Rid of the Bottom Bar on an iPhone?
Developers can write code that auto-hides the bar on some apps.
But, Apple has the final say.

There is one, big limitation: Guided Access works only in one app at a time.
You’ll have to trigger Guided Access for each app you open.
Its an essential child-proof feature to limit what kids can see and use on the screen.
Guided Access also works as a temporary fix for the bar at the bottom of the screen.
OpenSettings>Accessibility>Guided Access.
On theGuided Accessscreen, toggle the switch toOn.
pick the options that appear when Guided Access is enabled.
SelectSet Guided Access Passcodethat the iPhone will require to end a Guided Access session.
Accessibility shortcuts use a triple-hit the Power button to quickly access frequently used accessibility features.
pick the shortcuts from options > Accessibility > Accessibility Shortcuts.
initiate the app that you want to use without the Home Bar.
poke the Power button on the phone’s right side three times to activate Guided Access.
The iPhone enters the Guided Access mode.
Tap Guided Access and then tap again on Start.
To exit the Guided Access screen, triple-choose the Power button.
put in your Guided Access passcode, then tap End.
To use Face ID to exit Guided Access, double-poke the Side button.
Tip:
Siri is a quicker route to open a Guided Access session.
launch the app you want, then tell Siri to “Turn on Guided Access or Start Guided Access.
FAQ
First,download and install any available iOS updates.
Then, tapSettingsagain >Messages> turn off theShow Subject Fieldtoggle.
Clear History is grayed out when restrictions are on.
To turn off restrictions, go toSettings> tapScreen Timeand enable Screen Time if it’s not already enabled.
Then, in the Screen Time section, selectContent & Privacy Restrictions> turn off theContent and Privacy Restrictionstoggle.