Make sure App Store doesn't update apps over Mobile Data
There is no such mechanism of displaying an alert or notification when attempt is made to update apps over Mobile Data. (If you think about it, such a mechanism won't make any sense, as you may or may not notice the notification/alert when update is attempted over Mobile Data).

Your best bet is to turn off Mobile Data for automatic downloads. Go to Settings app → iTunes &amp; App Stores and turn off the switch for Use Mobile Data.



You can also choose to turn off Automatic Downloads of Updates, but that setting will apply to both Wi-Fi and Mobile Data.

Alternatively, you can turn on Low Power Mode by going to Settings app → Battery → Low Power Mode. This will turn off automatic downloading of Music, Apps and Updates. An active Low Power Mode is indicated by orange colour battery indicator in Status bar.



The unintended side effect of this approach would be that it will also turn off automatic download of updates over Wi-Fi.

Lastly, the best approach in my personal opinion would be to turn off Mobile Data for App Store app by going to Settings app → Mobile Data. This way there wouldn't be any intended or unintended downloads via App Store over Mobile Data. Also, this way you would be reminded when browsing the App Store, that you are not currently on Wi-Fi, even if you accidentally try to download or update an app.