Is index required for a PhD thesis? Nowadays, since (almost) all of the submitted works are usually available in an electronic format (e.g. PDF or Word) besides the printed version, is it recommended to add an index to a PhD thesis in order to facilitate search within the printed version? Was it required in the past?
You may if you want to, but it is not generally required. Of course, check your local regulations. It is recommended to plan on having an index or not when you start, rather than adding it later. 

Indexes were not required in the past, though check historical local regulations.