Nothing has changed about our support policy in many years (for any OS, but CentOS is particularly predictable). We support it for the entire life of the distribution. When CentOS 6 reaches EOL, we will also stop supporting it, but not before. This is the policy for every OS we support and has been for a long time.
Debian is just a little weird because they have a relatively new concept of “LTS” (with a very different meaning from the Ubuntu concept of “LTS”) where serious security issues will continue to be addressed in packages after the EOL of the release. Debian 8 reached EOL in 2018, but we kept supporting it on the assumption that we wouldn’t need to do anything to keep it working. But, Debian changed something in the Debian 8 repos (I think disabled one of the extra repositories) that broke our installer, and so the only reasonable option was to treat it like an EOL OS (which it is). So, I think we have to pretty much consider a Debian version dead when it reaches the Debian-announced EOL (even if it still gets security updates during the “LTS” period). Unfortunately, this means Debian may have too short of a lifecycle for people who are sensitive about having a long stable period for a new deployment.
Anyway, this is dictated by your OS of choice, not us. We can’t possibly maintain an entire operating system…so, when an OS version reaches EOL, we can no longer support it. If you have questions about EOL schedules, check with your OS provider.