Samsung is working on their upcoming foldable with new OneUI 3.1.1 and soon will roll out the update for the Galaxy S21 series. The new OneUI 3.1.1 will carry some minor changes and improvements to the existing OneUI 3.1.
TizenHelp has spotted one of the moderators talking about the new OneUI update. The moderator comments that the new OneUI update will start rolling out for the S21 series at the end of August, just after the August patch update. With no link to the actual post or comment, we cannot actually confirm with the news as of now. But if you see the update cycle every year, the new UI always takes a bit of time for the existing devices to get. Moreover, Samsung might also be working with Android 12 and their upcoming OneUI 4.0 as well.
Just because of this, do not take this as an actual piece of news and assume that the update will release in August.