It took a little bit longer than originally planned, but Raspberry Pi OS has received a major version upgrade to Debian 11, otherwise known as Bullseye. This is part of the two-year upgrade cycle for the Raspberry Pi OS, which prior to this point was based on Debian 10, otherwise known as Buster. Bullseye came out in August of this year.

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Source: Hot Hardware – Raspberry Pi OS Finally Hits Bullseye With Debian 11 Update Bringing Several Upgrades