Zoroastrians may be second to Hindus for the age of their religion, still practiced. As with any practice that spans millenniums, that comes with the caveat that the similarity between what it was when young and what it is now may lie mostly in the name and some cultural artifacts and in the imagination of those living.
It is dying. The proof of that is it now relies on matchmakers, in the hope that the next generation of faithful isn’t too much smaller than the current.
The analysis of 7th c. BC ostraca near the Dead Sea is interesting for its technique. I’m a bit leery of the conclusions about the spread of literacy. It’s easy to imagine in ancient times that literacy was a requirement for some military postings. And regardless of how widespread literacy was at the time, that has no direct bearing on how and when the Old Testament came to be. As with the Homeric sagas, it’s likely some parts of the Old Testament reflect older, oral stories. Christians typically and wrongly think of the Old Testament as being older than the Homeric stories, because they want to take the writing of the Old Testament as contemporary with the mythic history it presents. Of course, Gilgamesh predates them both by a dozen centuries or more.