Any way to get the double-tap full screen toggle back in Android 9?

I have a GS9+ that was running Android 8 up until this morning when I got an update pushed down from T-Mobile. I used to have a little dot icon in the bottom left corner that I could double-tap on to toggle between fullscreen mode and regular mode. That dot seems to be gone.

There are some apps that I sometimes use fullscreen and sometimes don't and I used that dot all the time. Does anyone know if there's a way to get that back?

Thanks.

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