No version of Pages for Mac, iCloud, or iOS has ever had the ability to generate an index. Only Table of Contents can be automatically generated based on applied Paragraph styles. If you want to generate an index, in a universally acceptable document format, then use the MS Word counterpart for Mac, cloud, or iOS.
Apple created Pages for consumers, and as such, it lacks many features that commercial (business) word processing applications include such as indexing. Wishing for Apple to add a commercial application feature is a waste of hope.