What does the letter I next to the Green Enabled Button mean? My iPhone shows this picture however, my wife's phone does not show that I?

Its not a letter I. Its just the universal symbol for being turned on much like they appear on physical switches:

As such you should see the O symbol on the other side of the switch when its turned off.

Why it does not show up on your wife's phone is probably because she has it turned off.

You can turn it off in Settings->Accessibility->Display and Text Size->On/Off labels.

Go to Settings>Accessibility>Display & Text Size>On/Off Labels. Turn that off. It will remove those markers.

It is common nomenclature for electronic devices, it means the switch is turned ON. A 0 would indicate it is off. Displaying this symbol is an option on the iPhone.

You are correct. I have as such corrected the post.