In a security updates post, Samsung lays out the devices that will get monthly, quarterly, and bi-annual updates. As recently spotted by 9to5Google, the S21 series will now be downgraded to quarterly updates.

Samsung's Galaxy S21 series, including the S21, S21 Plus, and S21 Ultra, will now receive quarterly updates instead of monthly updates.

In a move that we’re now seeing on a somewhat set schedule, Samsung’s Galaxy S21 series has been dropped to quarterly updates. With this, owners of the Galaxy S21, S21+, and S21 Ultra should no longer expect monthly security updates,

This will affect the Galaxy S21, S21+, and Galaxy S21 Ultra. But it will not have an effect on the Galaxy S21 FE, which was released a little later and is on a different schedule.