Epoch's don't break libraries - that's the whole point.
They manage this by allowing using a different epoch in a library than the application that uses it, and defaulting to the original epoch if you don't ask to use a new one.
The vast majority of rust 1.0 libraries should still work with modern rust programs, I vaguely recall that there was one minor backwards incompatible change as of a result of memory safety bug (an exception to the guarantee), but I can't remember what it was and I can't find it with google so it might actually be all rust 1.0 libraries still work.
They manage this by allowing using a different epoch in a library than the application that uses it, and defaulting to the original epoch if you don't ask to use a new one.
The vast majority of rust 1.0 libraries should still work with modern rust programs, I vaguely recall that there was one minor backwards incompatible change as of a result of memory safety bug (an exception to the guarantee), but I can't remember what it was and I can't find it with google so it might actually be all rust 1.0 libraries still work.