I always wondered about the security behind it, because it seemed like anything that hit our inbox that was a tiff and have a recognizable letterhead got acted on, no confirmations, 9-10 digit amounts were moving around based on that alone. Also Not sure that I saw the whole system or just a piece.
But just like the messaging layer, this can be done via other means. For example, here [2] is Citibank Poland declaring their reachability for various currency transfers.
As for the messaging service itself, I believe that cutting off SWIFT SSI access would help in disrupting payment flows in the short term, but in the long term, sanction lists are the real enforcement mechanism.
As others in this thread have already said, these are a very powerful indirect stick when implemented as "you must adhere to our embargo lists, or we will prohibit all of the banks in our jurisdiction to do any kind of business with you, whether for embargoed clients or others".
[1] https://www.swift.com/our-solutions/compliance-and-shared-se... [2] https://www.citibank.pl/poland/corporate/polish/files/list_o...
Look into correspondent banking if you're curious how all of that works – SWIFT is just one (very popular) way correspondent banks can communicate with each other and/or look up paths to facilitate international payments for their account holders.