Is there a recommended way to store a SSN in a contact?
Encrypted storage?
Only show a few digits after entry?
Etc.
I don’t know what an “SSN” is, but I understand you want to keep it secret 
There was this
https://github.com/salesagility/SuiteCRM/pull/1756
but it didn’t go anywhere because the encryption had to be upgraded, in order to be reliable, and then no-one else worked on it, I’m afraid…
You can use Jquery Mask to hide the input or a few characters with the Blur function.