I have just upgraded to Android 8.1 some days ago, and the "Scheduled power on and off" feature has been moved under "System" in the settings (as the whole settings menu has been reordered). Note, that the search also finds it for me. (type "scheduled").
So the feature is in place but does not work anymore, or at least my phone stopped to switch on since upgraded the Android version. Only the switch-off is actually working for me.
If you find the feature, I am curious whether both the power on and off will work for you.