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.
Whether its required is probably field/institution dependent. However, just for the sake of those who (we hope!) will be reading your thesis, include an index. Even in electronic versions, an index is a useful tool for the reader; it allows them to see what the author(s) thought important enough to index, if nothing else! And if your PhD thesis turns out to be good enough to be printed and bought by university libraries, you will definitely increase the ease of use for those who access the print version (and yes, some people, myself included, do still wander the physical stacks when researching!See this book for reasons why this is a good idea).
Ideally, I recommend having an index with hyperlinks (both from keywords on the text to the index, and the other way round). The difference with in-text search is that you select the most relevant occurrences of the selected words; this is especially useful with terms that you use at all pages, e.g. to point out where they are defined.
If you think it ahead, it is not that much work to do (at least with LaTeX and the right packages). Otherwise... I remember a few hours spend with two other people on this question for a PhD thesis in biology, where the index contained both abbreviations and complete names of a lot of molecules.