If you compare, say, C++03 and C++14, it's also technically true that "there's only new syntax", but in practice this often means that hacks that used to be idiomatic before are frowned upon now.
Anyway, there have been updated versions of the books and content online if people are interested.
Most changes have been in libraries and interpreter / VM implementation.
Updating your knowledge from Ruby 1.8 (mid 2000s) to 3.x (current) takes little effort.
But yes, sparse API documents were always a problem because a big chunk of the community was Japanese.