Some Galaxy S21, S21 + and S21 Ultra devices do not support MST for Samsung Pay. It seems that the availability of the function varies depending on the country – some receive it, others do not, and not in a way that makes a lot of sense.
MST allows the phone to work with sales terminals that only support old-school magnetic stripe cards. This is a big deal in the US, where such terminals are quite common, less so in regions where NFC terminals are plentiful.
Unfortunately, S21 phones sold in the US are in the group that does not have MST. Phones sold in the UK have it. Is it because of the different chipsets (Snapdragon in the US, Exynos in Europe)?
Here is Samsung’s statement on the issue asked by XDA:
Due to the rapid adoption of close communication technology (NFC) by consumers and businesses, starting with devices launched in 2021, Samsung Pay will focus its support on NFC transactions throughout the Galaxy portfolio. While future devices will no longer include magnetic stripe technology (MST), customers with previous, compatible Galaxy devices will be able to continue using Samsung Pay, including MST.
MST was introduced with the Galaxy S6 in 2015 and has been a major part of high-end galaxies ever since. He now has one foot through the door.