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/