In mid-September, Samsung started One UI 4.0 beta testing program for Galaxy S21 trio in supported countries. It is now live in all those countries that are selected for the beta program. Samsung has recently released a second beta version for Galaxy S21 series, and possibly this week, it will release the third beta version. It is almost one month since the company has started rolling out the beta version, and now both beta & non-beta users are asking when they will get a stable version. Looks like the team member at South Korean community has answered this question.
The Galaxy S community moderator has responded to the query and said the final version likely be ready in December. He says, Samsung is developing an Android 12-based One UI 4.0 stable version for the Galaxy S21 series, and the firmware likely roll out in the last month of this year. This answer is for KT operators in South Korea, so the SKT, LG U+ & Unlocked variants too get the version. Here is the official answer from Galaxy S community manager (machine translated).
Currently, Galaxy S21 is also developing the Android 12 (One UI 4) version and the situation is changing, so it is difficult to give a clear answer. We are actively collaborating with KT operators, and it is likely to be possible by December at the latest.
Previously, Samsung had surprised everyone by delivering stable One UI 3.0 for Galaxy S20 users in the US along with South Korea. That means there are chances this could also happen this year.
One UI 4.0 beta version still has a long way to go. Samsung still has to fix many issues with the beta version, so it could take time. The company still has to open a beta program for other flagship devices. In recent, the company has spotted building beta firmware for Galaxy S20 series & Galaxy Z series phones.