Whilst the four year old S21 series, (S21, S21+,S21 Ultra), are destined to receive the new Android 15/One UI 7.0 update in future, they will be downgraded to 3 monthly security updates.
As much as this is a bummer, 5 years of updates is still pretty good IMO.
I don't care really honestly about software updates, as for me, they usually bring unwanted and unnecessary changes. I care more about security updates.