Some Galaxy S21, S21+ and S21 Ultra units do not support MST for Samsung Pay. It looks like availability of the feature varies by country – some get it, others do not, and not in a way that makes much sense.
MST allows the phone to work with point of sale 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 abundant.
Unfortunately, the S21 phones sold in the US are in the group that does not have MST. Phones sold in the UK do have it. Is this because of the different chipsets (Snapdragon in the US, Exynos in Europe)?
That doesn’t seem to be the case. The spec sheets from the French, Italian and German press releases make no mention of MST. The Dutch one does list MST, as does the Polish, Russian and Indian press releases.
Here is Samsung’s statement on the matter when asked by XDA:
Due to the rapid adoption of near field communication (NFC) technology by consumers and businesses, beginning with devices launched in 2021, Samsung Pay will focus its support on NFC transactions, across 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 staple of high-end Galaxys ever since. Now it has one foot out the door.