Lineage purges old builds as they do not want to promote the use of outdated (and thus unsecure) roms. It is however totally possible to build all supported branches (16.0, 17.1, 18.1, 19.1) yourself, in which case the latest security patches from the upstream Android project get included automatically. Such builds are then "unofficial" in that they are not signed with the keys of the Lineage project, but otherwise they are based on the same source code.
@daniel.schaaaf maintains an unofficial lineage-17.1 rom for the pro1. See this thread.
Myself, I maintain an unofficial lineage-16