When users set (or don't set) ee8.preview.mode the effect thereof is very obscure. So we should log something, just to make it a bit clearer that the setting has been noticed.
I think the EE subsystem seems like a reasonable place to add this. Subsystem root add handler.
WFLY-10288 is the JIRA for cleaning out ee8.preview.mode behavior so I'll add reverting any change for this to the task list there.