I wouldn’t say it’s deprecated, but as José mentioned it is now several years old and targets Phoenix 1.4 - so if you want a book that covers more recent versions of Phoenix with all its latest goodies, such as LiveView (which most people probably will) then Programming Phoenix LiveView includes more of that recent stuff… until perhaps we see the next edition of Programming Phoenix.
PP 1.4 is also still worth getting if the topics it covers interest you. Personally, while I did not get around to finishing the book I really liked it - it basically shows you how to create a web framework, does an amazing job at selling Phoenix and includes loads of other interesting info too - such as about timing attacks! But despite all this, as you pointed out, most people coming to Phoenix will want a book that is more up-to-date, and right now, that’s Programming Phoenix LiveView (PragProg).
