It’s on the Fire Tablet OS. Mine (Fire HD 8 8thGen) is currently running 7.3.1.7 Build 0019227152516
I did read in one of the above articles that it is only available on newer models
" it’s only available on newer Amazon tablets, like the 2018 Fire HD 8, 2019 Fire 7, 2019 Fire HD 10, and 2020 Fire HD 8"