Is the Index still valid?
It no longer has the force of law, however all the books/authors that were on it at the time it was discontinued remain prohibited/discouraged. The Vatican clarified this in 1966 through the CDF. The moral obligation to of not circulating or reading the condemned materials remains.
Is it one of those things that nobody talks about but still exists in theory?
It does not merely exist “in theory”. It was replaced with a different procedure at the CDF
Why have the CDF stopped updating it?
Because with the explosion of media formats and worldwide publishing it became impossible to keep up with it in the manner that it had previously.
Additionally, after the reformation many authors were no longer under the jursidiction of the Catholic Church-- what good would it do to issue a censure of Joel Olsteen? Catholics must use some common sense and select books with the proper credentials, by Catholic authors, and also be aware of works specfically condemned by the CDF.
The CDF does keep doctrinal and disciplinary documents on their
website some with links to the full text.
The CDF in no way stopped issuing statements on books, authors, etc., they merely ammended their process to try to adapt to the overwhelming volume of literature (and new modes of communicaiton such as the web, TV, movies, etc) and the modern world.
