I feel the answer is obviously "yes", and indeed that much of 19th century mathematics was lost, in a serious sense, for much of the 20th century. I was struck recently by discovering that Henry Fox Talbot, the photographic pioneer, had written on what is clearly the area around Abel's theorem for curves, and that probably it is a long time since anyone reconstructed what he was doing. Also that George Boole's main work, as far as his contemporaries were concerned, dropped out of sight within a couple of decades.

The fact is that mathematics now is (a) axiomatic and (b) dominated by a canon. I'm reminded of Bertrand Russell's nightmare - where, a century after his death, the last copy of the Russell-Whitehead Principia Mathematica is in danger of being thrown out by an ignorant librarian. It actually isn't obvious that even such a pioneering work makes it into the mathematical logic "canon" around later developments. (I hear protests!) Maybe it is worth pointing out Hilbert's interest in Anschauliche Geometrie, in other words non-axiomatic, intuitive geometry. And the canon should be "porous", as has been argued by some of the Moscow school. It seems quite an illuminating take on mathematics as a living tradition that simple accretion of "known results" is misleading.