Even with math, there is much more "unwritten shared knowledge" than usually assumed. The corpus of current mathematics lives much more in the shared understanding of mathematicians than in the pages of articles. Math fields may indeed be lost if they are not actively practiced. There was a beautiful essay on this by Bill Thurston, that I don't have handy at the moment, but should be easy to find.

[EDIT:] maybe this: https://mathoverflow.net/questions/43690/whats-a-mathematici...