Interestingly enough, Ruby 1.8 -> 1.9, the big version jump there, there was this kind of transition. The remainder of this post is all IIRC, it's been a while...

Ruby 1.8 had "everything as bytes" and there was no concept of encodings.

Ruby 1.9 introduced explicit encodings on every string. By default, strings would be encoded as the same encoding as your source file. The default was ASCII. You could control this explicitly with a magic comment, and so many folks added the "UTF-8" comment, to get strings encoded as utf-8 by default.

Ruby 2.0, which was not as large a transition as Ruby 1.8 -> 1.9, even though it sounds like a larger one, said that encodings of files were UTF-8 by default, and therefore, strings generally became UTF-8 by default as well. Most folks just removed their magic comments.