The HN title is misleading as it was actually upgraded from Debian 8 "Jessie" to Bullseye.
Debian 8 "jessie" support reached its end-of-life on June 30, 2020, five years after its initial release on April 26, 2015.
However that does mean it was potentially vulnerable to critical vulnerabilities for more than two years whilst world+dog used it to download PuTTY for secure access to their servers. Eek
Would be good to know how this was managed if at all.
https://www.chiark.greenend.org.uk/~sgtatham/putty/latest.ht...