I was asked about W2 printing with masked SS# and this is what I found. I am hoping this saves someone the research.
This IRS article with updates in 2016 says it does not apply to W-2s but no cross reference to an article that does apply to W-2s (see last line in this article)
Current IRS instructions state…
Social security numbers. Do not truncate social security numbers shown on Forms W-2, W-2AS, W-2GU, and W-2VI. Social security numbers are required on Forms W-2. See Taxpayer identification numbers, later. See also Regulations section 301.6109-(4)(b)(2).
I also read 301.6109-(4)(b)(2) and based on the versions of the code I found one cannot truncate.