Or: The new version is reimplemented and incompatible, so everything you learnt about it from the previous versions is wrong.

For me its like "oh great a old textbook, now i can finally understand our legacy codebase".

One of the best programmers I've ever met told me, "All you need is Knuth everything else is just syntax." And I don't know if that's 100% true, but can say I learned more from reading The Art of Computer Programming than I have in basically any other textbook/textbook series I've read on the subject.

Or: "The algorithm and data structure theory stuff is still pretty relevant. However, all of the examples are written in a language no one really uses any more. If they can get away with it."