whenever there is a new major release version, there will be announcements as to when an older version will be discontinued. For e.g. when 5.0 released, it was mentioned that 3.x series will become EOL. See this blog as an example. Please also subscribe to mailing lists, https://cassandra.apache.org/_/community.html#discussions as well. Happy to learn if there are other locations where we could find this info.