If you are looking to set up Redmine today, here is the current landscape for finding and downloading the right installers for your environment. 1. The Current State: Virtual Machines & Containers
Some specific direct links still function. For example, the last published Linux x64 installer (version 4.2.1-3) can often be found at downloads.bitnami.com.
Sites like GitHub's mzbh/bitnami-installers repo host various older versions.
If you manage to secure a legacy installer, the process typically follows these steps: bitnami/redmine repository overview - Docker Hub