What is the earliest version that suppprts iPhone Wifi Assist equivalent for Android?
So apparently Google implemented it in 2014 in Android, and you couldn't properly disable it. Not even today.

I believe this feature is baked into Android.

Windows 8.1 definitely had it in 2014 according to this https://support.microsoft.com/en-us/help/10652/windows-phone-cellular-and-sim-settings I'm pretty sure Windows Phones predating this also had it although documentation seems lacking.

Wi-fi assist was introduced in iOS 9(2015). https://support.apple.com/en-us/HT205296

There are apps for Android that do this but at least this one is 3rd party. No sources says it predates 2015. I have not experimented enough with this on Android. https://www.guidingtech.com/54831/get-ios-like-wi-fi-assist-android-using-speedify/