In an effort to help protect individual identities from theft, the IRS has recently proposed permitting truncated Social Security numbers on W-2s. Employers would be voluntarily allowed to use truncated SSNs on the copies of Form W-2 distributed to employees. The numbers would appear as IRS truncated taxpayer identification numbers (TTINs).
The proposal also states that employees who report third-party sick pay are allowed to have their SSN truncated on their W-2 copy, and employees that receive wages in the form of group-term life insurance are allowed the same.
You can read the full proposal here. The IRS is allowing public comments on these proposed regulations until December 18, 2017.