Starting in that last two weeks (I successfully installed cassandra sometime in the last two weeks), I'm guessing on 2/19 when version 3.11.2 was released, the cassandra apt package version 3.11.1 became unstable. It doesn't seem to be published in the http://www.apache.org/dist/cassandra/debian repository anymore (at least not in a valid state).
Despite the package actually being in the repository still http://www.apache.org/dist/cassandra/debian/pool/main/c/cassandra/cassandra_3.11.1_all.deb It is no longer in the Packages list http://dl.bintray.com/apache/cassandra/dists/311x/main/binary-amd64/Packages It looks like version 3.11.2 was released on 2/19. I'm guessing that publishing dropped the 3.11.1 version from the packages list.
