Magnetic Secure Transmission, (MST), is a contactless payment solution that emulates the magstripe terminals still used by many credit card readers in the U.S.

The bad news is that it will not be available to U.S. users on the S21 series or on future releases.

However, strangely enough, some other countries will still be able to use the MST technology as this article from xda developers explains...

Samsung kills MST: the US Galaxy S21 won’t support Samsung Pay’s best payment method

The U.K, Ireland, The Netherlands, Poland, Russia and India are among those that will continue to feature MST.